What is the meaning of the word time zone

A time zone is an area which observes a uniform standard time for legal, commercial and social purposes. Time zones tend to follow the boundaries between countries and their subdivisions instead of strictly following longitude, because it is convenient for areas in frequent communication to keep the same time.

All time zones are defined as offsets from Coordinated Universal Time (UTC), ranging from UTC−12:00 to UTC+14:00. The offsets are usually a whole number of hours, but a few zones are offset by an additional 30 or 45 minutes, such as in India, South Australia and Nepal.

Some areas of higher latitude use daylight saving time for about half of the year, typically by adding one hour to local time during spring and summer.

List of UTC offsets

In the table below, the locations that use daylight saving time (DST) are listed in their UTC offset when DST is not in effect. When DST is in effect, approximately during spring and summer, their UTC offset is increased by one hour (except for Lord Howe Island, where it is increased by 30 minutes). For example, during the DST period California observes UTC−07:00 and the United Kingdom observes UTC+01:00.

UTC offset	Locations that do not use DST	Locations that use DST
UTC−12:00		Baker Island  Howland Island
UTC−11:00		American Samoa  Jarvis Island  Kingman Reef  Midway Atoll  Niue  Palmyra Atoll
UTC−10:00		Cook Islands  French Polynesia (most)  Johnston Atoll  United States: Hawaii		United States: Andreanof Islands, Islands of Four Mountains, Near Islands, Rat Islands (Aleutian Islands, Alaska)
UTC−09:30		French Polynesia: Marquesas Islands
UTC−09:00		French Polynesia: Gambier Islands		United States: Alaska (most)
UTC−08:00		Clipperton Island  Pitcairn Islands		Canada: British Columbia (most)  Mexico: Baja California  United States: California, Idaho (north), Nevada (most), Oregon (most), Washington
UTC−07:00		Canada: British Columbia (northeast), Yukon  Mexico: Baja California Sur, Nayarit (most), Sinaloa, Sonora  United States: Arizona (most)		Canada: Alberta, British Columbia (southeast), Northwest Territories, Nunavut (west)  Mexico: Chihuahua (northwest border)  United States: Arizona (Navajo Nation), Colorado, Idaho (most), Kansas (west), Montana, Nebraska (west), New Mexico, Nevada (northeast border), North Dakota (southwest), Oregon (east), South Dakota (west), Texas (west), Utah, Wyoming
UTC−06:00		Belize  Canada: Saskatchewan (most)  Costa Rica  Ecuador: Galápagos  El Salvador  Guatemala  Honduras  Mexico (most)  Nicaragua		Canada: Manitoba, Nunavut (central), Ontario (west)  Chile: Easter Island  Mexico (northeast border)  United States: Alabama, Arkansas, Florida (northwest), Illinois, Indiana (northwest, southwest), Iowa, Kansas (most), Kentucky (west), Louisiana, Michigan (northwest border), Minnesota, Mississippi, Missouri, Nebraska (most), North Dakota (most), Oklahoma, South Dakota (most), Tennessee (most), Texas (most), Wisconsin
UTC−05:00		Brazil: Acre, Amazonas (southwest)  Canada: Atikokan, Mishkeegogamang, Southampton Island  Cayman Islands  Colombia  Ecuador (most)  Jamaica  Mexico: Quintana Roo  Navassa Island  Panama  Peru		Bahamas  Canada: Nunavut (east), Ontario (most), Quebec (most)  Cuba  Haiti  Turks and Caicos Islands  United States: Connecticut, Delaware, District of Columbia, Florida (most), Georgia, Indiana (most), Kentucky (most), Maine, Maryland, Massachusetts, Michigan (most), New Hampshire, New Jersey, New York, North Carolina, Ohio, Pennsylvania, Rhode Island, South Carolina, Tennessee (east), Vermont, Virginia, West Virginia
UTC−04:00		Anguilla  Antigua and Barbuda  Aruba  Barbados  Bolivia  Brazil: Amazonas (most), Mato Grosso, Mato Grosso do Sul, Rondônia, Roraima  British Virgin Islands  Canada: Quebec (east)  Caribbean Netherlands  Curaçao  Dominica  Dominican Republic  Grenada  Guadeloupe  Guyana  Martinique  Montserrat  Puerto Rico  Saint Barthélemy  Saint Kitts and Nevis  Saint Lucia  Saint Martin  Saint Vincent and the Grenadines  Sint Maarten  Trinidad and Tobago  U.S. Virgin Islands  Venezuela		Bermuda  Canada: Labrador (most), New Brunswick, Nova Scotia, Prince Edward Island  Chile (most)  Greenland: Pituffik Space Base  Paraguay
UTC−03:30				Canada: Newfoundland, Labrador (southeast)
UTC−03:00		Argentina  Brazil (most)  Chile: Magallanes Region  Falkland Islands  French Guiana  Suriname  Uruguay		Saint Pierre and Miquelon
UTC−02:00		Brazil: Fernando de Noronha  Greenland (most)  South Georgia and the South Sandwich Islands
UTC−01:00		Cape Verde		Greenland: Ittoqqortoormiit  Portugal: Azores
UTC+00:00		Burkina Faso  Gambia  Ghana  Greenland: National Park (east coast)  Guinea  Guinea-Bissau  Iceland  Ivory Coast  Liberia  Mali  Mauritania  Saint Helena, Ascension and Tristan da Cunha  Senegal  Sierra Leone  São Tomé and Príncipe  Togo		Faroe Islands  Guernsey  Ireland  Isle of Man  Jersey  Portugal (most)  Spain: Canary Islands  United Kingdom
UTC+01:00		Algeria  Angola  Benin  Cameroon  Central African Republic  Chad  Congo  Democratic Republic of the Congo: Équateur, Kinshasa, Kongo Central, Kwango, Kwilu, Mai-Ndombe, Mongala, Nord-Ubangi, Sud-Ubangi, Tshuapa  Equatorial Guinea  Gabon  Morocco[a]  Niger  Nigeria  Tunisia  Western Sahara[a]		Albania  Andorra  Austria  Belgium  Bosnia and Herzegovina  Croatia  Czech Republic  Denmark  France (metropolitan)  Germany  Gibraltar  Hungary  Italy  Kosovo  Liechtenstein  Luxembourg  Malta  Monaco  Montenegro  Netherlands (European)  North Macedonia  Norway  Poland  San Marino  Serbia  Slovakia  Slovenia  Spain (most)  Sweden  Switzerland   Vatican City
UTC+02:00		Botswana  Burundi  Democratic Republic of the Congo (most)  Eswatini  Lesotho  Libya  Malawi  Mozambique  Namibia  Russia: Kaliningrad  Rwanda  South Africa (most)  South Sudan  Sudan  Zambia  Zimbabwe		Akrotiri and Dhekelia  Bulgaria  Cyprus  Egypt  Estonia  Finland  Greece  Israel  Latvia  Lebanon  Lithuania  Moldova  Northern Cyprus  Palestine  Romania  Transnistria  Ukraine (most)
UTC+03:00		Abkhazia  Bahrain  Belarus  Comoros  Djibouti  Eritrea  Ethiopia  French Southern and Antarctic Lands: Scattered Islands[4]  Iraq  Jordan  Kenya  Kuwait  Madagascar  Mayotte  Qatar  Russia (most of European part)  Saudi Arabia  Somalia  Somaliland  South Africa: Prince Edward Islands  South Ossetia  Syria  Tanzania  Turkey  Uganda  Ukraine: occupied territories  Yemen
UTC+03:30		Iran
UTC+04:00		Armenia  Artsakh  Azerbaijan  French Southern and Antarctic Lands: Crozet Islands[4]  Georgia  Mauritius  Oman  Russia: Astrakhan, Samara, Saratov, Udmurtia, Ulyanovsk  Réunion  Seychelles  United Arab Emirates
UTC+04:30		Afghanistan
UTC+05:00		French Southern and Antarctic Lands: Kerguelen Islands, Saint Paul Island, Amsterdam Island  Heard Island and McDonald Islands  Kazakhstan: Aktobe, Atyrau, Baikonur, Kyzylorda, Mangystau, West Kazakhstan  Maldives  Pakistan  Russia: Bashkortostan, Chelyabinsk, Khanty-Mansi, Kurgan, Orenburg, Perm, Sverdlovsk, Tyumen, Yamalia  Tajikistan  Turkmenistan  Uzbekistan
UTC+05:30		India  Sri Lanka
UTC+05:45		Nepal
UTC+06:00		Bangladesh  Bhutan  British Indian Ocean Territory  Kazakhstan (most)  Kyrgyzstan  Russia: Omsk
UTC+06:30		Cocos Islands  Myanmar
UTC+07:00		Cambodia  Christmas Island  Indonesia: Sumatra, Java, West Kalimantan, Central Kalimantan  Laos  Mongolia: Bayan-Ölgii, Khovd, Uvs  Russia: Altai Krai, Altai Republic, Kemerovo, Khakassia, Krasnoyarsk, Novosibirsk, Tomsk, Tuva  Thailand  Vietnam
UTC+08:00		Australia: Western Australia (most)  Brunei  China  Hong Kong  Indonesia: South Kalimantan, East Kalimantan, North Kalimantan, Sulawesi, Lesser Sunda Islands  Macau  Malaysia  Mongolia (most)  Philippines  Russia: Buryatia, Irkutsk  Singapore  Taiwan
UTC+08:45		Australia: Eucla
UTC+09:00		East Timor  Indonesia: Maluku Islands, Western New Guinea  Japan  North Korea  Palau  Russia: Amur, Sakha (most), Zabaykalsky  South Korea
UTC+09:30		Australia: Northern Territory		Australia: South Australia
UTC+10:00		Australia: Queensland  Guam  Micronesia: Chuuk, Yap  Northern Mariana Islands  Papua New Guinea (most)  Russia: Jewish, Khabarovsk, Primorsky, Sakha (central-east)		Australia: Australian Capital Territory, Jervis Bay Territory, New South Wales (most), Tasmania, Victoria
UTC+10:30				Australia: Lord Howe Island
UTC+11:00		Micronesia: Kosrae, Pohnpei  New Caledonia  Papua New Guinea: Bougainville  Russia: Magadan, Sakha (east), Sakhalin  Solomon Islands  Vanuatu		Norfolk Island
UTC+12:00		Fiji  Kiribati: Gilbert Islands  Marshall Islands  Nauru  Russia: Chukotka, Kamchatka  Tuvalu  Wake Island  Wallis and Futuna		New Zealand (most)
UTC+12:45				New Zealand: Chatham Islands
UTC+13:00		Kiribati: Phoenix Islands  Samoa  Tokelau  Tonga
UTC+14:00		Kiribati: Line Islands

History

The apparent position of the Sun in the sky, and thus solar time, varies by location due to the spherical shape of the Earth. This variation corresponds to four minutes of time for every degree of longitude, so for example when it is solar noon in London, it is about 10 minutes before solar noon in Bristol, which is about 2.5 degrees to the west.^[5]

The Royal Observatory, Greenwich, founded in 1675, established Greenwich Mean Time (GMT), the mean solar time at that location, as an aid to mariners to determine longitude at sea, providing a standard reference time while each location in England kept a different time.

Railway time

In the 19th century, as transportation and telecommunications improved, it became increasingly inconvenient for each location to observe its own solar time. In November 1840, the Great Western Railway started using GMT kept by portable chronometers.^[6] This practice was soon followed by other railway companies in Great Britain and became known as Railway Time.

Around August 23, 1852, time signals were first transmitted by telegraph from the Royal Observatory. By 1855, 98% of Great Britain’s public clocks were using GMT, but it was not made the island’s legal time until August 2, 1880. Some British clocks from this period have two minute hands, one for the local time and one for GMT.^[7]

On November 2, 1868, the then British Colony of New Zealand officially adopted a standard time to be observed throughout the colony.^[8] It was based on longitude 172°30′ east of Greenwich, that is 11 hours 30 minutes ahead of GMT. This standard was known as New Zealand Mean Time.^[9]

Timekeeping on North American railroads in the 19th century was complex. Each railroad used its own standard time, usually based on the local time of its headquarters or most important terminus, and the railroad’s train schedules were published using its own time. Some junctions served by several railroads had a clock for each railroad, each showing a different time.^[10]

Charles F. Dowd proposed a system of hourly standard time zones for North American railroads around 1863, although he published nothing on the matter at that time and did not consult railroad officials until 1869. In 1870 he proposed four ideal time zones having north–south borders, the first centered on Washington, D.C., but by 1872 the first was centered on meridian 75° west of Greenwich, with natural borders such as sections of the Appalachian Mountains. Dowd’s system was never accepted by North American railroads. Instead, U.S. and Canadian railroads implemented a version proposed by William F. Allen, the editor of the Traveler’s Official Railway Guide.^[11] The borders of its time zones ran through railroad stations, often in major cities. For example, the border between its Eastern and Central time zones ran through Detroit, Buffalo, Pittsburgh, Atlanta, and Charleston. It was inaugurated on Sunday, November 18, 1883, also called «The Day of Two Noons»,^[12] when each railroad station clock was reset as standard-time noon was reached within each time zone.

The North American zones were named Intercolonial, Eastern, Central, Mountain, and Pacific. Within a year 85% of all cities with populations over 10,000 (about 200 cities) were using standard time.^[13] A notable exception was Detroit (located about halfway between the meridians of Eastern and Central time), which kept local time until 1900, then tried Central Standard Time, local mean time, and Eastern Standard Time (EST) before a May 1915 ordinance settled on EST and was ratified by popular vote in August 1916. The confusion of times came to an end when standard time zones were formally adopted by the U.S. Congress in the Standard Time Act of March 19, 1918.

Worldwide time zones

Italian mathematician Quirico Filopanti introduced the idea of a worldwide system of time zones in his book Miranda!, published in 1858. He proposed 24 hourly time zones, which he called «longitudinal days», the first centred on the meridian of Rome. He also proposed a universal time to be used in astronomy and telegraphy. However, his book attracted no attention until long after his death.^[14][15]

Scottish-born Canadian Sir Sandford Fleming proposed a worldwide system of time zones in 1876 — see Sandford Fleming § Inventor of worldwide standard time. The proposal divided the world into twenty-four time zones labeled A-Y (skipping J), each one covering 15 degrees of longitude. All clocks within each zone would be set to the same time as the others, but differed by one hour from those in the neighboring zones.^[16] He advocated his system at several international conferences, including the International Meridian Conference, where it received some consideration. The system has not been directly adopted, but some maps divide the world into 24 time zones and assign letters to them, similarly to Fleming’s system.^[17]

By about 1900, almost all inhabited places on Earth had adopted a standard time zone, but only some of them used an hourly offset from GMT. Many applied the time at a local astronomical observatory to an entire country, without any reference to GMT. It took many decades before all time zones were based on some standard offset from GMT or Coordinated Universal Time (UTC). By 1929, the majority of countries had adopted hourly time zones, though some countries such as Iran, India, Myanmar and parts of Australia had time zones with a 30-minute offset. Nepal was the last country to adopt a standard offset, shifting slightly to UTC+05:45 in 1986.^[18]

All nations currently use standard time zones for secular purposes, but not all of them apply the concept as originally conceived. Several countries and subdivisions use half-hour or quarter-hour deviations from standard time. Some countries, such as China and India, use a single time zone even though the extent of their territory far exceeds the ideal 15° of longitude for one hour; other countries, such as Spain and Argentina, use standard hour-based offsets, but not necessarily those that would be determined by their geographical location. The consequences, in some areas, can affect the lives of local citizens, and in extreme cases contribute to larger political issues, such as in the western reaches of China.^[19] In Russia, which has 11 time zones, two time zones were removed in 2010^[20][21] and reinstated in 2014.^[22]

Notation

ISO 8601

ISO 8601 is a standard established by the International Organization for Standardization defining methods of representing dates and times in textual form, including specifications for representing time zones.^[23]

If a time is in Coordinated Universal Time (UTC), a «Z» is added directly after the time without a separating space. «Z» is the zone designator for the zero UTC offset. «09:30 UTC» is therefore represented as «09:30Z» or «0930Z». Likewise, «14:45:15 UTC» is written as «14:45:15Z» or «144515Z».^[24] UTC time is also known as «Zulu» time, since «Zulu» is a phonetic alphabet code word for the letter «Z».^[24]

Offsets from UTC are written in the format ±hh:mm, ±hhmm, or ±hh (either hours ahead or behind UTC). For example, if the time being described is one hour ahead of UTC (such as the time in Germany during the winter), the zone designator would be «+01:00», «+0100», or simply «+01». This numeric representation of time zones is appended to local times in the same way that alphabetic time zone abbreviations (or «Z», as above) are appended. The offset from UTC changes with daylight saving time, e.g. a time offset in Chicago, which is in the North American Central Time Zone, is «−06:00» for the winter (Central Standard Time) and «−05:00» for the summer (Central Daylight Time).^[25]

Abbreviations

Time zones are often represented by alphabetic abbreviations such as «EST», «WST», and «CST», but these are not part of the international time and date standard ISO 8601. Such designations can be ambiguous; for example, «CST» can mean (North American) Central Standard Time (UTC−06:00), Cuba Standard Time (UTC−05:00) and China Standard Time (UTC+08:00), and it is also a widely used variant of ACST (Australian Central Standard Time, UTC+09:30).^[26]

Conversions

Conversion between time zones obeys the relationship

«time in zone A» − «UTC offset for zone A» = «time in zone B» − «UTC offset for zone B»,

in which each side of the equation is equivalent to UTC.

The conversion equation can be rearranged to

«time in zone B» = «time in zone A» − «UTC offset for zone A» + «UTC offset for zone B».

For example, the New York Stock Exchange opens at 09:30 (EST, UTC offset= −05:00). In California (PST, UTC offset= −08:00) and India (IST, UTC offset= +05:30), the New York Stock Exchange opens at

time in California = 09:30 − (−05:00) + (−08:00) = 06:30;

time in India = 09:30 − (−05:00) + (+05:30) = 20:00.

These calculations become more complicated near the time switch to or from daylight saving time, as the UTC offset for the area becomes a function of UTC time.

The time differences may also result in different dates. For example, when it is 22:00 on Monday in Egypt (UTC+02:00), it is 01:00 on Tuesday in Pakistan (UTC+05:00).

The table «Time of day by zone» gives an overview on the time relations between different zones.

Time of day by zone
UTC offset	Monday
UTC−12:00	00:00	01:00	02:00	03:00	04:00	05:00	06:00	07:00	08:00	09:00	10:00	11:00	12:00	13:00	14:00	15:00	16:00	17:00	18:00	19:00	20:00	21:00	22:00	23:00
UTC−11:00	01:00	02:00	03:00	04:00	05:00	06:00	07:00	08:00	09:00	10:00	11:00	12:00	13:00	14:00	15:00	16:00	17:00	18:00	19:00	20:00	21:00	22:00	23:00	00:00
UTC−10:00	02:00	03:00	04:00	05:00	06:00	07:00	08:00	09:00	10:00	11:00	12:00	13:00	14:00	15:00	16:00	17:00	18:00	19:00	20:00	21:00	22:00	23:00	00:00	01:00
UTC−09:30	02:30	03:30	04:30	05:30	06:30	07:30	08:30	09:30	10:30	11:30	12:30	13:30	14:30	15:30	16:30	17:30	18:30	19:30	20:30	21:30	22:30	23:30	00:30	01:30
UTC−09:00	03:00	04:00	05:00	06:00	07:00	08:00	09:00	10:00	11:00	12:00	13:00	14:00	15:00	16:00	17:00	18:00	19:00	20:00	21:00	22:00	23:00	00:00	01:00	02:00
UTC−08:00	04:00	05:00	06:00	07:00	08:00	09:00	10:00	11:00	12:00	13:00	14:00	15:00	16:00	17:00	18:00	19:00	20:00	21:00	22:00	23:00	00:00	01:00	02:00	03:00
UTC−07:00	05:00	06:00	07:00	08:00	09:00	10:00	11:00	12:00	13:00	14:00	15:00	16:00	17:00	18:00	19:00	20:00	21:00	22:00	23:00	00:00	01:00	02:00	03:00	04:00
UTC−06:00	06:00	07:00	08:00	09:00	10:00	11:00	12:00	13:00	14:00	15:00	16:00	17:00	18:00	19:00	20:00	21:00	22:00	23:00	00:00	01:00	02:00	03:00	04:00	05:00
UTC−05:00	07:00	08:00	09:00	10:00	11:00	12:00	13:00	14:00	15:00	16:00	17:00	18:00	19:00	20:00	21:00	22:00	23:00	00:00	01:00	02:00	03:00	04:00	05:00	06:00
UTC−04:00	08:00	09:00	10:00	11:00	12:00	13:00	14:00	15:00	16:00	17:00	18:00	19:00	20:00	21:00	22:00	23:00	00:00	01:00	02:00	03:00	04:00	05:00	06:00	07:00
UTC−03:30	08:30	09:30	10:30	11:30	12:30	13:30	14:30	15:30	16:30	17:30	18:30	19:30	20:30	21:30	22:30	23:30	00:30	01:30	02:30	03:30	04:30	05:30	06:30	07:30
UTC−03:00	09:00	10:00	11:00	12:00	13:00	14:00	15:00	16:00	17:00	18:00	19:00	20:00	21:00	22:00	23:00	00:00	01:00	02:00	03:00	04:00	05:00	06:00	07:00	08:00
UTC−02:30	09:30	10:30	11:30	12:30	13:30	14:30	15:30	16:30	17:30	18:30	19:30	20:30	21:30	22:30	23:30	00:30	01:30	02:30	03:30	04:30	05:30	06:30	07:30	08:30
UTC−02:00	10:00	11:00	12:00	13:00	14:00	15:00	16:00	17:00	18:00	19:00	20:00	21:00	22:00	23:00	00:00	01:00	02:00	03:00	04:00	05:00	06:00	07:00	08:00	09:00
UTC−01:00	11:00	12:00	13:00	14:00	15:00	16:00	17:00	18:00	19:00	20:00	21:00	22:00	23:00	00:00	01:00	02:00	03:00	04:00	05:00	06:00	07:00	08:00	09:00	10:00
UTC+00:00	12:00	13:00	14:00	15:00	16:00	17:00	18:00	19:00	20:00	21:00	22:00	23:00	00:00	01:00	02:00	03:00	04:00	05:00	06:00	07:00	08:00	09:00	10:00	11:00
UTC+01:00	13:00	14:00	15:00	16:00	17:00	18:00	19:00	20:00	21:00	22:00	23:00	00:00	01:00	02:00	03:00	04:00	05:00	06:00	07:00	08:00	09:00	10:00	11:00	12:00
UTC+02:00	14:00	15:00	16:00	17:00	18:00	19:00	20:00	21:00	22:00	23:00	00:00	01:00	02:00	03:00	04:00	05:00	06:00	07:00	08:00	09:00	10:00	11:00	12:00	13:00
UTC+03:00	15:00	16:00	17:00	18:00	19:00	20:00	21:00	22:00	23:00	00:00	01:00	02:00	03:00	04:00	05:00	06:00	07:00	08:00	09:00	10:00	11:00	12:00	13:00	14:00
UTC+03:30	15:30	16:30	17:30	18:30	19:30	20:30	21:30	22:30	23:30	00:30	01:30	02:30	03:30	04:30	05:30	06:30	07:30	08:30	09:30	10:30	11:30	12:30	13:30	14:30
UTC+04:00	16:00	17:00	18:00	19:00	20:00	21:00	22:00	23:00	00:00	01:00	02:00	03:00	04:00	05:00	06:00	07:00	08:00	09:00	10:00	11:00	12:00	13:00	14:00	15:00
UTC+04:30	16:30	17:30	18:30	19:30	20:30	21:30	22:30	23:30	00:30	01:30	02:30	03:30	04:30	05:30	06:30	07:30	08:30	09:30	10:30	11:30	12:30	13:30	14:30	15:30
UTC+05:00	17:00	18:00	19:00	20:00	21:00	22:00	23:00	00:00	01:00	02:00	03:00	04:00	05:00	06:00	07:00	08:00	09:00	10:00	11:00	12:00	13:00	14:00	15:00	16:00
UTC+05:30	17:30	18:30	19:30	20:30	21:30	22:30	23:30	00:30	01:30	02:30	03:30	04:30	05:30	06:30	07:30	08:30	09:30	10:30	11:30	12:30	13:30	14:30	15:30	16:30
UTC+05:45	17:45	18:45	19:45	20:45	21:45	22:45	23:45	00:45	01:45	02:45	03:45	04:45	05:45	06:45	07:45	08:45	09:45	10:45	11:45	12:45	13:45	14:45	15:45	16:45
UTC+06:00	18:00	19:00	20:00	21:00	22:00	23:00	00:00	01:00	02:00	03:00	04:00	05:00	06:00	07:00	08:00	09:00	10:00	11:00	12:00	13:00	14:00	15:00	16:00	17:00
UTC+06:30	18:30	19:30	20:30	21:30	22:30	23:30	00:30	01:30	02:30	03:30	04:30	05:30	06:30	07:30	08:30	09:30	10:30	11:30	12:30	13:30	14:30	15:30	16:30	17:30
UTC+07:00	19:00	20:00	21:00	22:00	23:00	00:00	01:00	02:00	03:00	04:00	05:00	06:00	07:00	08:00	09:00	10:00	11:00	12:00	13:00	14:00	15:00	16:00	17:00	18:00
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UTC+11:00	23:00	00:00	01:00	02:00	03:00	04:00	05:00	06:00	07:00	08:00	09:00	10:00	11:00	12:00	13:00	14:00	15:00	16:00	17:00	18:00	19:00	20:00	21:00	22:00
UTC+12:00	00:00	01:00	02:00	03:00	04:00	05:00	06:00	07:00	08:00	09:00	10:00	11:00	12:00	13:00	14:00	15:00	16:00	17:00	18:00	19:00	20:00	21:00	22:00	23:00
UTC+12:45	00:45	01:45	02:45	03:45	04:45	05:45	06:45	07:45	08:45	09:45	10:45	11:45	12:45	13:45	14:45	15:45	16:45	17:45	18:45	19:45	20:45	21:45	22:45	23:45
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UTC+14:00	02:00	03:00	04:00	05:00	06:00	07:00	08:00	09:00	10:00	11:00	12:00	13:00	14:00	15:00	16:00	17:00	18:00	19:00	20:00	21:00	22:00	23:00	00:00	01:00
UTC offset	Tuesday	Wednesday

Nautical time zones

Since the 1920s, a nautical standard time system has been in operation for ships on the high seas. As an ideal form of the terrestrial time zone system, nautical time zones consist of gores of 15° offset from GMT by a whole number of hours. A nautical date line follows the 180th meridian, bisecting one 15° gore into two 7.5° gores that differ from GMT by ±12 hours.^[27][28][29]

However, in practice each ship may choose what time to observe at each location. Ships may decide to adjust their clocks at a convenient time, usually at night, not exactly when they cross a certain longitude.^[30] Some ships simply remain on the time of the departing port during the whole trip.^[31]

Skewing of time zones

Ideal time zones, such as nautical time zones, are based on the mean solar time of a particular meridian located in the middle of that zone with boundaries located 7.5 degrees east and west of the meridian. In practice, however, many time zone boundaries are drawn much farther to the west, and some countries are located entirely outside their ideal time zones.

For example, even though the Prime Meridian (0°) passes through Spain and France, they use the mean solar time of 15 degrees east (Central European Time) rather than 0 degrees (Greenwich Mean Time). France previously used GMT, but was switched to CET (Central European Time) during the German occupation of the country during World War II and did not switch back after the war.^[32] Similarly, prior to World War II, the Netherlands observed «Amsterdam Time», which was twenty minutes ahead of Greenwich Mean Time. They were obliged to follow German time during the war, and kept it thereafter. In the mid-1970s the Netherlands, as other European states, began observing daylight saving (summer) time.

One reason to draw time zone boundaries far to the west of their ideal meridians is to allow the more efficient use of afternoon sunlight.^[33] Some of these locations also use daylight saving time (DST), further increasing the difference to local solar time. As a result, in summer, solar noon in the Spanish city of Vigo occurs at 14:41 clock time. This westernmost area of continental Spain never experiences sunset before 18:00 clock time, even in winter, despite lying 42 degrees north of the equator.^[34] Near the summer solstice, Vigo has sunset times after 22:00, similar to those of Stockholm, which is in the same time zone and 17 degrees farther north. Stockholm has much earlier sunrises, though.^[35]

A more extreme example is Nome, Alaska, which is at 165°24′W longitude – just west of center of the idealized Samoa Time Zone (165°W). Nevertheless, Nome observes Alaska Time (135°W) with DST so it is slightly more than two hours ahead of the sun in winter and over three in summer.^[36]
Kotzebue, Alaska, also near the same meridian but north of the Arctic Circle, has two sunsets on the same day in early August, one shortly after midnight at the start of the day, and the other shortly before midnight at the end of the day.^[37]

China extends as far west as 73°E, but all parts of it use UTC+08:00 (120°E), so solar «noon» can occur as late as 15:00 in western portions of China such as Xinjiang.^[38] The Afghanistan-China border marks the greatest terrestrial time zone difference on Earth, with a 3.5 hour difference between Afghanistan’s UTC+4:30 and China’s UTC+08:00.

Daylight saving time

Many countries, and sometimes just certain regions of countries, adopt daylight saving time (DST), also known as summer time, during part of the year. This typically involves advancing clocks by an hour near the start of spring and adjusting back in autumn («spring forward», «fall back»). Modern DST was first proposed in 1907 and was in widespread use in 1916 as a wartime measure aimed at conserving coal. Despite controversy, many countries have used it off and on since then; details vary by location and change occasionally. Countries around the equator usually do not observe daylight saving time, since the seasonal difference in sunlight there is minimal.

Computer systems

Many computer operating systems include the necessary support for working with all (or almost all) possible local times based on the various time zones. Internally, operating systems typically use UTC as their basic time-keeping standard, while providing services for converting local times to and from UTC, and also the ability to automatically change local time conversions at the start and end of daylight saving time in the various time zones. (See the article on daylight saving time for more details on this aspect).

Web servers presenting web pages primarily for an audience in a single time zone or a limited range of time zones typically show times as a local time, perhaps with UTC time in brackets. More internationally oriented websites may show times in UTC only or using an arbitrary time zone. For example, the international English-language version of CNN includes GMT and Hong Kong Time,^[39] whereas the US version shows Eastern Time.^[40] US Eastern Time and Pacific Time are also used fairly commonly on many US-based English-language websites with global readership. The format is typically based in the W3C Note «datetime».

Email systems and other messaging systems (IRC chat, etc.)^[41] time-stamp messages using UTC, or else include the sender’s time zone as part of the message, allowing the receiving program to display the message’s date and time of sending in the recipient’s local time.

Database records that include a time stamp typically use UTC, especially when the database is part of a system that spans multiple time zones. The use of local time for time-stamping records is not recommended for time zones that implement daylight saving time because once a year there is a one-hour period when local times are ambiguous.

Calendar systems nowadays usually tie their time stamps to UTC, and show them differently on computers that are in different time zones. That works when having telephone or internet meetings. It works less well when travelling, because the calendar events are assumed to take place in the time zone the computer or smartphone was on when creating the event. The event can be shown at the wrong time. For example, if a New Yorker plans to meet someone in Los Angeles at 9 AM, and makes a calendar entry at 9 AM (which the computer assumes is New York time), the calendar entry will be at 6 AM if taking the computer’s time zone. There is also an option in newer versions of Microsoft Outlook to enter the time zone in which an event will happen, but often not in other calendar systems. Calendaring software must also deal with daylight saving time (DST). If, for political reasons, the begin and end dates of daylight saving time are changed, calendar entries should stay the same in local time, even though they may shift in UTC time. In Microsoft Outlook, time stamps are therefore stored and communicated without DST offsets.^[42] Hence, an appointment in London at noon in the summer will be represented as 12:00 (UTC+00:00) even though the event will actually take place at 13:00 UTC. In Google Calendar, calendar events are stored in UTC (although shown in local time) and might be changed by a time-zone changes,^[43] although normal daylight saving start and end are compensated for (similar to much other calendar software).

Operating systems

Unix

Most Unix-like systems, including Linux and Mac OS X, keep system time in time_t format, representing the number of seconds (excluding leap seconds) that have elapsed since 00:00:00 Coordinated Universal Time (UTC) on Thursday, January 1, 1970.^[44] By default the external representation is as UTC (Coordinated Universal Time), though individual processes can specify time zones using the TZ environment variable.^[45] This allows users in multiple time zones to use the same computer, with their respective local times displayed correctly to each user. Time zone information most commonly comes from the IANA time zone database. In fact, many systems, including anything using the GNU C Library, can make use of this database.

Microsoft Windows

Windows-based computer systems prior to Windows 2000 used local time, but Windows 2000 and later can use UTC as the basic system time.^[46] The system registry contains time zone information that includes the offset from UTC and rules that indicate the start and end dates for daylight saving in each zone. Interaction with the user normally uses local time, and application software is able to calculate the time in various zones. Terminal Servers allow remote computers to redirect their time zone settings to the Terminal Server so that users see the correct time for their time zone in their desktop/application sessions. Terminal Services uses the server base time on the Terminal Server and the client time zone information to calculate the time in the session.

Programming languages

Java

While most application software will use the underlying operating system for time zone information, the Java Platform, from version 1.3.1, has maintained its own time zone database. This database is updated whenever time zone rules change. Oracle provides an updater tool for this purpose.^[47]

As an alternative to the time zone information bundled with the Java Platform, programmers may choose to use the Joda-Time library.^[48] This library includes its own time zone data based on the IANA time zone database.^[49]

As of Java 8 there is a new date and time API that can help with converting time zones.
Java 8 Date Time

JavaScript

Traditionally, there was very little in the way of time zone support for JavaScript. Essentially the programmer had to extract the UTC offset by instantiating a time object, getting a GMT time from it, and differencing the two. This does not provide a solution for more complex daylight saving variations, such as divergent DST directions between northern and southern hemispheres.

ECMA-402, the standard on Internationalization API for JavaScript, provides ways of formatting Time Zones.^[50] However, due to size constraint, some implementations or distributions do not include it.^[51]

Perl

The DateTime object in Perl supports all time zones in the Olson DB and includes the ability to get, set and convert between time zones.^[52]

PHP

The DateTime objects and related functions have been compiled into the PHP core since 5.2. This includes the ability to get and set the default script time zone, and DateTime is aware of its own time zone internally. PHP.net provides extensive documentation on this.^[53] As noted there, the most current time zone database can be implemented via the PECL timezonedb.

Python

The standard module datetime included with Python stores and operates on the time zone information class tzinfo. The third party pytz module provides access to the full IANA time zone database.^[54] Negated time zone offset in seconds is stored time.timezone and time.altzone attributes. From Python 3.9, the zoneinfo module introduces timezone management without need for third party module.^[55]

Smalltalk

Each Smalltalk dialect comes with its own built-in classes for dates, times and timestamps, only a few of which implement the DateAndTime and Duration classes as specified by the ANSI Smalltalk Standard. VisualWorks provides a TimeZone class that supports up to two annually recurring offset transitions, which are assumed to apply to all years (same behavior as Windows time zones). Squeak provides a Timezone class that does not support any offset transitions. Dolphin Smalltalk does not support time zones at all.

For full support of the tz database (zoneinfo) in a Smalltalk application (including support for any number of annually recurring offset transitions, and support for different intra-year offset transition rules in different years) the third-party, open-source, ANSI-Smalltalk-compliant Chronos Date/Time Library is available for use with any of the following Smalltalk dialects: VisualWorks, Squeak, Gemstone, or Dolphin.^[56]

Time in outer space

Orbiting spacecraft may experience many sunrises and sunsets, or none, in a 24-hour period. Therefore, it is not possible to calibrate the time with respect to the Sun and still respect a 24-hour sleep/wake cycle. A common practice for space exploration is to use the Earth-based time of the launch site or mission control, synchronizing the sleeping cycles of the crew and controllers. The International Space Station normally uses Greenwich Mean Time (GMT).^[57][58]

Timekeeping on Mars can be more complex, since the planet has a solar day of approximately 24 hours and 40 minutes, known as a sol. Earth controllers for some Mars missions have synchronized their sleep/wake cycles with the Martian day, because solar-powered rover activity on the surface was tied to periods of light and dark.^[59]

See also

• Daylight saving time
• ISO 8601
• Jet lag
• Lists of time zones
• Metric time
• Time by country
• Time in Europe
• Time zone abolition
• World clock
• International Date Line

Notes

References

Further reading

• Biswas, Soutik (February 12, 2019). «How India’s single time zone is hurting its people». BBC News. Retrieved February 12, 2019.
• Maulik Jagnani, economist at Cornell University (January 15, 2019). «PoorSleep: Sunset Time and Human Capital Production» (Job Market Paper). Retrieved February 12, 2019.
• «Time Bandits: The countries rebelling against GMT» (Video). BBC News. August 14, 2015. Retrieved February 12, 2019.
• «How time zones confused the world». BBC News. August 7, 2015. Retrieved February 12, 2019.
• Lane, Megan (May 10, 2011). «How does a country change its time zone?». BBC News. Retrieved February 12, 2019.
• «A brief history of time zones» (Video). BBC News. March 24, 2011. Retrieved February 12, 2019.
• The Time Zone Information Format (TZif). doi:10.17487/RFC8536. RFC 8536.

External links

• Media related to Time zones at Wikimedia Commons

Полезное

Last Updated:
12 Apr 2023
— Updated example sentences

Asked by: Ms. Alize Hickle PhD

Score: 4.3/5
(31 votes)

A time zone is an area that observes a uniform standard time for legal, commercial and social purposes. Time zones tend to follow the boundaries between countries and their subdivisions instead of strictly following longitude, because it is convenient for areas in frequent communication to keep the same time.

What is the meaning of zone time?

: standard time applied at sea in which the surface of the globe is divided into 24 zones of 15° or one hour each, the 0 zone extends 7¹/₂° east and west of the meridian of Greenwich, and the zones are designated by the number of hours that must be applied to the local time to obtain Greenwich Mean Time.

What does what is your timezone mean?

: a geographic region within which the same standard time is used.

What is a timezone give an example?

The term time zone is often used instead of local time. For instance, during DST, it is common to say “California and Arizona are now in the same time zone.” However, the correct thing to say would be: “California and Arizona now have the same local time.”

What is zone with example?

verb. 1. The definition of a zone is an area, region or section which is used or restricted for a specified purpose. An example of a zone is an area where the typical weather allows specific vegetables to grow well there. An example of a zone is a section of the highway where construction is happening.

43 related questions found

This article is about time zones in general. For a list of time zones by country, see List of time zones by country. For more time zone lists, see Lists of time zones.

A time zone is a region on Earth that has a uniform standard time for legal, commercial, and social purposes. In order for the same clock time to always correspond to the same portion of the day as the Earth rotates (for example, the sun being at its highest point every day around noon), different places on the Earth need to have different clock times. Time zones have been used in modern times so similarly situated cities can keep exactly the same time, for simplicity and ease of communication.

Standard time zones could be defined by geometrically subdividing the Earth’s spheroid into 24 lunes (wedge-shaped sections), bordered by meridians each 15° of longitude apart. The local time in neighboring zones would differ by one hour, and the variation in the position of the sun from one end of the zone to the other (east vs. west) would be at most 1/24th of the sky. Most of the 25 nautical time zones (specifically UTC−11 to UTC+11) are indeed defined this way, and are 15° of longitude wide. An hourly zone in the central Pacific Ocean is split into two 7.5° wide zones (UTC±12) by the 180th meridian, part of which coincides with the International Date Line.

On land, it is more convenient for areas in close commercial or other communication to keep the same time, so time zones tend to follow the boundaries of countries and their subdivisions instead. Of the 40 time zones on land, most are offset from Coordinated Universal Time (UTC) by a whole number of hours (UTC−12 to UTC+14), but a few are offset by 30 or 45 minutes from a nearby hourly zone. Daylight saving time is used in some higher-latitude countries to manipulate clock time with respect to the position of the sun for parts of the year, typically by changing clocks by an hour. Many land time zones are skewed toward the west relative to the corresponding nautical time zones, which also creates a permanent daylight saving time-like offset. Computer operating systems use either UTC or a local time zone to time stamp events.

History

Early timekeeping

Before the invention of clocks, people marked the time of day with apparent solar time (or «true» solar time) — for example, the time on a sundial — which was typically different for every settlement.

When well-regulated mechanical clocks became widespread in the early 19th century,^{[citation needed]} each city began to use some local mean solar time. Apparent and mean solar time can differ by up to around 15 minutes (as described by the equation of time) due to the non-circular shape of the Earth’s orbit around the sun. Mean solar time has days of equal length, and the difference between the two averages to zero after a year.

Greenwich Mean Time (GMT) was established in 1675 when the Royal Observatory was built as an aid to (English) mariners to determine longitude at sea, providing a reference time at a point in history when each city in England kept a different local time.

Railroad time

The use of local solar time became increasingly awkward as railways and telecommunications improved, because clocks differed between places by an amount corresponding to the difference in their geographical longitude, which was usually not a convenient number.

The first time zone in the world was established on December 1, 1847, on the island of Great Britain by railway companies using GMT kept by portable chronometers. This quickly became known as Railway Time. About August 23, 1852, time signals were first transmitted by telegraph from the Royal Observatory, Greenwich. Even though 98% of Great Britain’s public clocks were using GMT by 1855, it was not made Britain’s legal time until August 2, 1880. Some old British clocks from this period have two minute hands—one for the local time, one for GMT.^[1]

The increase in worldwide communication has further increased the need for interacting parties to communicate mutually comprehensible time references to one another. The problem of differing local times could be solved across larger areas by synchronizing clocks worldwide, but in many places the local time would then differ markedly from the solar time to which people were accustomed. Time zones are a compromise, relaxing the complex geographic dependence while still allowing local time to approximate the mean solar time.

On November 2, 1868, the then-British colony of New Zealand officially adopted a standard time to be observed throughout the colony, and was perhaps the first country to do so. It was based on the longitude 172°30′ East of Greenwich, that is 11 hours 30 minutes ahead of GMT. This standard was known as New Zealand Mean Time.

Timekeeping on the American railroads in the mid-19th century was somewhat confused. Each railroad used its own standard time, usually based on the local time of its headquarters or most important terminus, and the railroad’s train schedules were published using its own time. Some major railroad junctions served by several different railroads had a separate clock for each railroad, each showing a different time; the main station in Pittsburgh, Pennsylvania, for example, kept six different times.

Charles F. Dowd proposed a system of one-hour standard time zones for American railroads about 1863, although he published nothing on the matter at that time and did not consult railroad officials until 1869. In 1870, he proposed four ideal time zones (having north–south borders), the first centered on Washington, D.C., but by 1872 the first was centered 75°W of Greenwich, with geographic borders (for example, sections of the Appalachian Mountains). Dowd’s system was never accepted by American railroads. Instead, U.S. and Canadian railroads implemented a version proposed by William F. Allen, the editor of the Traveler’s Official Railway Guide.^[2] The borders of its time zones ran through railroad stations, often in major cities. For example, the border between its Eastern and Central time zones ran through Detroit, Buffalo, Pittsburgh, Atlanta, and Charleston. It was inaugurated on Sunday, November 18, 1883, also called «The Day of Two Noons»,^[3] when each railroad station clock was reset as standard-time noon was reached within each time zone. The zones were named Intercolonial, Eastern, Central, Mountain, and Pacific. Within one year, 85% of all cities with populations over 10,000, about 200 cities, were using standard time.^[4] A notable exception was Detroit (which is about half-way between the meridians of eastern time and central time), which kept local time until 1900, then tried Central Standard Time, local mean time, and Eastern Standard Time before a May 1915 ordinance settled on EST and was ratified by popular vote in August 1916. The confusion of times came to an end when Standard zone time was formally adopted by the U.S. Congress on March 19, 1918, in the Standard Time Act.

U.S. Commissioner of Railroads William H. Armstrong gave the following account of the new railroad time system in his Report to the Secretary of the Interior for 1883.

Worldwide time zones

Although the first person to propose a worldwide system of time zones was the Italian mathematician Quirico Filopanti in his book Miranda! published in 1858, his idea was unknown outside the pages of his book until long after his death, so it did not influence the adoption of time zones during the 19th century. He proposed 24 hourly time zones, which he called «longitudinal days», the first centered on the meridian of Rome. He also proposed a universal time to be used in astronomy and telegraphy.^[8][9]

Canadian Sir Sandford Fleming proposed a worldwide system of time zones in 1879. He advocated his system at several international conferences, thus is widely credited with their invention. In 1876, his first proposal was for a global 24-hour clock, conceptually located at the center of the Earth and not linked to any surface meridian. In 1879 he specified that his universal day would begin at the anti-meridian of Greenwich (180th meridian), while conceding that hourly time zones might have some limited local use. He also proposed his system at the International Meridian Conference in October 1884, but it did not adopt his time zones because they were not within its purview. The conference did adopt a universal day of 24 hours beginning at Greenwich midnight, but specified that it «shall not interfere with the use of local or standard time where desirable».

By about 1900, almost all time on Earth was in the form of standard time zones, only some of which used an hourly offset from GMT. Many applied the time at a local astronomical observatory to an entire country, without any reference to GMT. It took many decades before all time on Earth was in the form of time zones referred to some «standard offset» from GMT/UTC. Most major countries had adopted hourly time zones by 1929. Nepal was the last country to adopt a standard offset, shifting slightly to UTC+5:45 in 1986.

Today, all nations use standard time zones for secular purposes, but they do not all apply the concept as originally conceived. Newfoundland, India, Iran, Afghanistan, Venezuela, Burma, the Marquesas, as well as parts of Australia use half-hour deviations from standard time, and some nations, such as Nepal, and some provinces, such as the Chatham Islands, use quarter-hour deviations. Some countries, most notably China and India, use a single time zone, even though the extent of their territory far exceeds 15° of longitude. Before 1949, China used five time zones (see Time in China).

Definition

Before 1972, all time zones were specified as an offset from Greenwich Mean Time (GMT), which was the mean solar time at the meridian passing through the Royal Observatory in Greenwich, London, United Kingdom. Since 1972, all official time services have broadcast radio time signals synchronized to UTC, a form of atomic time that includes leap seconds to keep it within 0.9 seconds of this former GMT, now called UT1. Many countries now legally define their standard time relative to UTC, although some still legally refer to GMT, including the United Kingdom itself. UTC, also called Zulu time, is used everywhere on Earth by astronomers and others who need to state the time of an event unambiguously.

Time zones are based on Greenwich Mean Time (GMT)^{[citation needed]}, the mean solar time at longitude 0° (the Prime Meridian). The definition of GMT was recently changed^{[citation needed]} – it was previously the same as UT1, a mean solar time calculated directly from the rotation of the Earth. As the rate of rotation of the Earth is not constant, the time derived from atomic clocks was adjusted to closely match UT1. In January 1972, however, the length of the second in both Greenwich Mean Time and atomic time was equalized. The readings of participating atomic clocks are averaged out to give a uniform time scale.

Because the length of the average day is a small fraction of a second more than 24 hours (slightly more than 86400 seconds), leap seconds are periodically inserted into Greenwich Mean Time to make it approximate to UT1. This new time system is also called Coordinated Universal Time (UTC). Leap seconds are inserted to keep UTC within 0.9 seconds of UT1. Because of the secular (long term) slowing down of the Earth’s rotation, leap seconds will gradually need to be added more and more often. But on short time scales (from one year to the next) the rotation rate is irregular, so leap seconds are not added unless observations of Earth’s rotation show that one is required. In this way, local times continue to correspond approximately to mean solar time, while the effects of variations in Earth’s rotation rate are confined to simple step changes that can be more easily applied to the uniform time scale (International Atomic Time or TAI). All local times differ from TAI by an integral number of seconds. With the implementation of UTC, nations began to use it in the definition of their time zones. As of 2005, most nations had altered the definition of local time in this way.

In the United Kingdom, this involved redefining Greenwich Mean Time to make it the same as UTC.^[10] British Summer Time (BST) is still one hour in advance of Greenwich Mean Time and is therefore also one hour in advance of Coordinated Universal Time. Thus Greenwich Mean Time is the local time at the Royal Observatory, Greenwich between 0100 hours GMT on the last Sunday in October and 0100 hours GMT on the last Sunday in March. Similar circumstances apply in many other places.

Looking to the future, leap seconds are considered by many to be a nuisance, and ways to abolish them are being considered. This means letting the time difference accumulate. One suggestion is to insert a «leap-hour» in about 5,000 years. For more on this discussion read leap second#Proposal to abolish leap seconds.

Notation of time

ISO 8601

UTC

If the time is in UTC, add a «Z» directly after the time without a space. «Z» is the zone designator for the zero UTC offset. «09:30 UTC» is therefore represented as «09:30Z» or «0930Z». «14:45:15 UTC» would be «14:45:15Z» or «144515Z».

UTC time is also known as «Zulu» time, since «Zulu» is the ICAO spelling alphabet code word for «Z».

Offsets from UTC

Offsets from UTC are written in the format ±[hh]:[mm], ±[hh][mm], or ±[hh]. So if the time being described is one hour ahead of UTC (such as the time in Berlin during the winter), the zone designator would be «+01:00», «+0100», or simply «+01». This is appended to the time in the same way that ‘Z’ was above. The offset from UTC changes with daylight saving time, e.g. a time offset in Chicago, which is in the North American Central Time Zone, would be «−06:00» for the winter (Central Standard Time) and «−05:00» for the summer (Central Daylight Time).

Abbreviations

Time zones are often represented by abbreviations such as «EST, WST, CST» but these are not part of the international time and date standard ISO 8601 and their use as sole designator for a time zone is not recommended. Such designations can be ambiguous. For example, «BST», which is British Summer Time, was renamed «British Standard Time» between 1968 and 1971 when Central European Time was in force because legislators objected to calling it Central European Time. The same legislation affirmed that the Standard Time within the United Kingdom was, and would continue to be, Greenwich Mean Time.

UTC offsets worldwide

Main article: List of time zones by UTC offset

Time zones map (Last update: October 18, 2011)

UTC−12:00 … UTC−07:00 UTC−06:00 … UTC−01:00 UTC±00:00 … UTC+05:45 UTC+06:00 … UTC+11:30 UTC+12:00 … UTC+14:00 Oceania / North America / Antarctica North and South America / Antarctica Europe / Africa / Asia / Antarctica Asia / Antarctica Asia / Oceania / Antarctica No DST in summer DST in summer No DST in summer DST in summer No DST in summer DST in summer No DST in summer DST in summer No DST in summer DST in summer −12:00 −12:00 /−11:00 N: US- −06:00 −06:00 /−05:00 N: US- ±00:00 IS ±00:00 /+01:00 N: GB, PT +06:00 RU-, KZ— +06:00 /+07:00 +12:00 KI-, RU- +12:00 /+13:00 S: NZ- +06:30 MM +06:30 /+07:30 +12:45 +12:45 /+13:45 S: NZ −11:00 −11:00 /−10:00 N: US- −05:00 BO, CO, PA, PE −05:00 /−04:00 N: CA-, CU, US- +01:00 TN, CG, CD-, DZ, NE, NG +01:00 /+02:00 N: AT, BA, BE, CH, CZ, DE, ES-, FR, HR, HU, IT, LI, LU, MK, NL, NO, SE, SI, SK S: NA +07:00 RU-, MN-, VN, LA, TH, KH, ID- +07:00 /+08:00 +13:00 KI- −04:30 VE −10:00 −10:00 /−09:00 −04:00 −04:00 /−03:00 S: AQ- +02:00 Europe: BY +02:00 /+03:00 N: FI, EE, LV, LT, UA, BG, GR, MD, RO, TR +08:00 CN, HK, ID-, MN-, MY, RU-, SG, TW, +08:00 /+09:00 +14:00 KI- −03:30 −03:30 /−02:30 S: CA- −09:00 −09:00 /−08:00 −03:00 AR −03:00 /−02:00 S: BR- +03:00 Europe: BY, RU-, Africa: KE, SD, SO, SS, ER, Asia: IQ, SA +03:00 /+04:00 +09:00 RU-, JP, KP, KR, ID- +09:00 /+10:00 +03:30 +03:30 /+04:30 IR +09:30 +09:30 /+10:30 AU- −08:00 −08:00 /−07:00 N: CA-, US-, MX- −02:00 −02:00 /−01:00 +04:00 RU-, GE +04:00 /+05:00 +10:00 RU- +10:00 /+11:00 +04:30 AF −07:00 −07:00 /−06:00 N: CA-, US-, MX- −01:00 −01:00 /±00:00 +05:00 KZ-, PK +05:00 /+06:00 +11:00 RU- +11:00 /+12:00 +05:30 IN +11:30 NF +05:45 NP XX = ISO 3166-1 alpha-2 country code, XX- = parts of the country, N = North, S = South, UTC = Universal Coordinated Time, DST = Daylight Saving Time

List of UTC offsets

These examples give the local time at various locations around the world when daylight saving time is not in effect:

Time zone	Example time (ISO 8601 notation)	Example locations	Example locations that in summer use DST
UTC−12:00	2011-11-20T06:02:43-12:00	Baker Island, Howland Island (both uninhabited)
UTC−11:00	2011-11-20T07:02:43-11:00	American Samoa, Samoa
UTC−10:00	2011-11-20T08:02:43-10:00		French Polynesia (most), United States (Aleutian Islands, Hawaii)
UTC−09:30	2011-11-20T08:32:43-09:30	Marquesas Islands
UTC−09:00	2011-11-20T09:02:43-09:00	Gambier Islands	United States (most of Alaska)
UTC−08:00	2011-11-20T10:02:43-08:00		Canada (most of British Columbia), Mexico (Baja California), United States (California, most of Nevada, most of Oregon, Washington (state))
UTC−07:00	2011-11-20T11:02:43-07:00	Mexico (Sonora), United States (Arizona)	Canada (Alberta), Mexico (Chihuahua), United States (Colorado)
UTC−06:00	2011-11-20T12:02:43-06:00	Costa Rica, El Salvador, Ecuador (Galápagos Islands), Guatemala, Honduras, Mexico (most), Nicaragua,	Canada (Manitoba, Saskatchewan), United States (Illinois, most of Texas)
UTC−05:00	2011-11-20T13:02:43-05:00	Colombia, Cuba, Ecuador (continental), Haiti, Jamaica, Panama, Peru	Canada (most of Ontario, most of Quebec), United States (most of Florida, Georgia, Massachusetts, most of Michigan, New York, North Carolina, Ohio, Washington D.C.)
UTC−04:30	2011-11-20T13:32:43-04:30	Venezuela
UTC−04:00	2011-11-20T14:02:43-04:00	Bolivia, Brazil (Amazonas), Chile (continental), Dominican Republic, Canada (Nova Scotia), Paraguay, Puerto Rico, Trinidad and Tobago
UTC−03:30	2011-11-20T14:32:43-03:30	Canada (Newfoundland)
UTC−03:00	2011-11-20T15:02:43-03:00	Argentina	Brazil (Brasília, Rio de Janeiro, São Paulo), most of Greenland, Uruguay
UTC−02:00	2011-11-20T16:02:43-02:00	Brazil (Fernando de Noronha), South Georgia and the South Sandwich Islands
UTC−01:00	2011-11-20T17:02:43-01:00	Portugal (Azores), Cape Verde
UTC±00:00	2011-11-20T18:02:43+00:00	Côte d’Ivoire, Faroe Islands, Ghana, Iceland, Senegal	Ireland, Portugal (continental and Madeira), Spain (Canary Islands), Morocco, United Kingdom
UTC+01:00	2011-11-20T19:02:43+01:00	Angola, Cameroon, Nigeria, Tunisia	Albania, Algeria, Austria, Belgium, Bosnia and Herzegovina, Spain (continental), Croatia, Czech Republic, Denmark, Germany, Hungary, Italy, Kinshasa, Kosovo, Macedonia, France (metropolitan), the Netherlands, Norway, Poland, Serbia, Slovakia, Slovenia, Sweden, Switzerland
UTC+02:00	2011-11-20T20:02:43+02:00	Libya, Egypt, Malawi, Mozambique, South Africa, Zambia, Zimbabwe	Bulgaria, Cyprus, Estonia, Finland, Greece, Israel, Jordan, Latvia, Lebanon, Lithuania, Moldova, Palestine, Romania, Syria, Turkey, Ukraine
UTC+03:00	2011-11-20T21:02:43+03:00	Belarus, Djibouti, Eritrea, Ethiopia, Iraq, Kenya, Madagascar, Russia (Kaliningrad Oblast), Saudi Arabia, South Sudan, Sudan, Somalia, South Sudan, Tanzania, Uganda, Yemen
UTC+03:30	2011-11-20T21:32:43+03:30		Iran
UTC+04:00	2011-11-20T22:02:43+04:00	Armenia, Azerbaijan, Georgia, Mauritius, Oman, Russia (European), Seychelles, United Arab Emirates
UTC+04:30	2011-11-20T22:32:43+04:30	Afghanistan
UTC+05:00	2011-11-20T23:02:43+05:00	Kazakhstan (West), Maldives, Pakistan, Uzbekistan
UTC+05:30	2011-11-20T23:32:43+05:30	India, Sri Lanka
UTC+05:45	2011-11-20T23:47:43+05:45	Nepal
UTC+06:00	2011-11-21T00:02:43+06:00	Kazakhstan (most), Bangladesh, Russia (Ural: Sverdlovsk Oblast, Chelyabinsk Oblast)
UTC+06:30	2011-11-21T00:32:43+06:30	Cocos Islands, Myanmar
UTC+07:00	2011-11-21T01:02:43+07:00	Jakarta, Russia (Novosibirsk Oblast), Thailand, Vietnam
UTC+08:00	2011-11-21T02:02:43+08:00	China, Hong Kong, Russia (Krasnoyarsk Krai), Malaysia, Philippines, Singapore, Taiwan, most of Mongolia, Western Australia
UTC+09:00	2011-11-21T03:02:43+09:00	East Timor, Russia (Irkutsk Oblast), Japan, North Korea, South Korea
UTC+09:30	2011-11-21T03:32:43+09:30	Australia (Northern Territory)	Australia (South Australia)
UTC+10:00	2011-11-21T04:02:43+10:00	Russia (Zabaykalsky Krai)	Australia (New South Wales, Queensland, Tasmania, Victoria)
UTC+10:30	2011-11-21T04:32:43+10:30	Lord Howe Island
UTC+11:00	2011-11-21T05:02:43+11:00	New Caledonia, Russia (Primorsky Krai), Solomon Islands
UTC+11:30	2011-11-21T05:32:43+11:30	Norfolk Island
UTC+12:00	2011-11-21T06:02:43+12:00	Fiji, Russia (Kamchatka Krai)	New Zealand
UTC+12:45	2011-11-21T06:47:43+12:45		Chatham Islands
UTC+13:00	2011-11-21T07:02:43+13:00	Tonga
UTC+14:00	2011-11-21T08:02:43+14:00	Kiribati (Line Islands)

Where the adjustment for time zones results in a time at the other side of midnight from UTC, then the date at the location is one day later or earlier.

Some examples when UTC is 23:00 on Monday when or where daylight saving time is not in effect:

• Cairo, Egypt: UTC+02; 01:00 on Tuesday
• Wellington, New Zealand: UTC+12; 11:00 on Tuesday

Some examples when UTC is 02:00 on Tuesday when or where daylight saving time is not in effect:

• Honolulu, Hawaii, United States: UTC−10; 16:00 on Monday
• Toronto, Ontario, Canada: UTC−05; 21:00 on Monday

The time-zone adjustment for a specific location may vary because of daylight saving time. For example New Zealand, which is usually UTC+12, observes a one-hour daylight saving time adjustment during the Southern Hemisphere summer, resulting in a local time of UTC+13.

Time zone conversions

Conversion between time zones obeys the relationship

«time in zone A» − «UTC offset for zone A» = «time in zone B» − «UTC offset for zone B»,

in which each side of the equation is equivalent to UTC. (The more familiar term «UTC offset» is used here rather than the term «zone designator» used by the standard.)

The conversion equation can be rearranged to

«time in zone B» = «time in zone A» − «UTC offset for zone A» + «UTC offset for zone B».

For example, what time is it in Los Angeles (UTC offset= −08) when the New York Stock Exchange opens at 09:30 (−05)?

time in Los Angeles = 09:30 − (−05:00) + (−08:00) = 06:30

In Delhi (UTC offset= +5:30), the New York Stock Exchange opens at

time in Delhi = 09:30 − (−05:00) + (+5:30) = 20:00

These calculations become more complicated near a daylight saving boundary (because the UTC offset for zone X is a function of the UTC time).

Nautical time zones

Since the 1920s a nautical standard time system has been in operation for ships on the high seas. Nautical time zones are an ideal form of the terrestrial time zone system. Under the system, a time change of one hour is required for each change of longitude by 15°. The 15° gore that is offset from GMT or UT1 (not UTC) by twelve hours is bisected by the nautical date line into two 7.5° gores that differ from GMT by ±12 hours. A nautical date line is implied but not explicitly drawn on time zone maps. It follows the 180th meridian except where it is interrupted by territorial waters adjacent to land, forming gaps: it is a pole-to-pole dashed line.^[11][12][13]

A ship within the territorial waters of any nation would use that nation’s standard time, but would revert to nautical standard time upon leaving its territorial waters. The captain is permitted to change the ship’s clocks at a time of the captain’s choice following the ship’s entry into another time zone. The captain often chooses midnight. Ships going in shuttle traffic over a time zone border often keeps the same time zone all the time, to avoid confusion about work, meal and shop opening hours. Still the time table for port calls must follow the land time zone.

Skewing of zones

Ideal time zones, such as nautical time zones, are based on the mean solar time of a particular meridian located in the middle of that zone with boundaries located 7.5 degrees east and west of the meridian. In practice, zone boundaries are often drawn much farther to the west with often irregular boundaries, and some locations base their time on meridians located far to the east.

For example, even though the Prime Meridian (0°) passes through Spain and France, they use the mean solar time of 15 degrees east (Central European Time) rather than 0 degrees (Greenwich Mean Time). France previously used GMT, but was switched to CET (Central European Time) during the German occupation of the country during World War II and did not switch back after the war.^{[citation needed]}

There is a tendency to draw time zone boundaries far to the west of their meridians. Many of these locations also use daylight saving time. As a result, in the summer, solar noon in the Spanish town of Muxia occurs at 14:37 (2:37pm) by the clock. This area of Spain never experiences sunset before 18:00 (6pm) local time even in midwinter, despite its lying more than 40 degrees north of the equator. Near the summer solstice, Muxia has sunset times similar to those of Stockholm, which is in the same time zone and 16 degrees further north.

A more extreme example is Nome, Alaska, which is at 165°24′W longitude—just west of center of the idealized Samoa Time Zone (165°W). Nevertheless, Nome observes Alaska Time (135°W) with DST so it is slightly more than two hours ahead of the sun in winter and over three in summer.^[14] Kotzebue, Alaska, also near the same meridian but north of the Arctic Circle, has an annual event on 9 August to celebrate two sunsets in the same 24-hour day, one shortly after midnight at the start of the day, and the other shortly before midnight at the end of the day.

Also, China extends as far west as 73°34′E, but all parts of it use UTC+08:00 (120°E), so solar «noon» can occur as late as 15:00.

Daylight saving time

Many countries, and sometimes just certain regions of countries, adopt daylight saving time (also known as «Summer Time») during part of the year. This typically involves advancing clocks by an hour near the start of spring and adjusting back in autumn («spring» forward, «fall» back). Some countries also use backward daylight saving over the winter period. Modern DST was first proposed in 1907 and was in widespread use in 1916 as a wartime measure aimed at conserving coal. Despite controversy, many countries have used it since then; details vary by location and change occasionally. Most countries around the equator do not observe daylight saving time, since the seasonal difference in sunlight is minimal.

Additional information

• France has twelve time zones including those of Metropolitan France, French Guiana and numerous islands, inhabited and uninhabited. Russia has nine time zones (and used to have 11 time zones before March 2010), eight contiguous zones plus Kaliningrad exclave on the Baltic Sea. The United States has ten time zones (nine official plus that for Wake Island and its Antarctic stations; no official time zone is specified for uninhabited Howland Island and Baker Island). Australia has nine time zones (one unofficial and three official on the mainland plus four for its territories and one more for an Antarctic station not included in other time zones). The United Kingdom has eight time zones for itself and its overseas territories. Canada has six official time zones. The Danish Realm has five time zones.
• In terms of area, China is the largest country with only one time zone (UTC+08). China also has the widest spanning time zone. Before 1949, China was separated into five time zones.
• Stations in Antarctica generally keep the time of their supply bases, thus both the Amundsen-Scott South Pole Station (U.S.) and McMurdo Station (U.S.) use New Zealand time (UTC+12 southern winter, UTC+13 southern summer).
• The 27°N latitude passes back and forth across time zones in South Asia. Pakistan: +05:00, India +05:30, Nepal +05:45, +05:30, Myanmar +06:30. This switching was more odd in 2002, when Pakistan enabled daylight saving time. Thus from west to east, time zones were: +06:00, +05:30, +05:45, +05:30, +08:00, +06:00, +05:30 and +06:30.
• Because the earliest and latest time zones are 26 hours apart, any given calendar date exists at some point on the globe for 50 hours. For example, April 11 begins in time zone UTC+14 at 10:00 UTC April 10, and ends in time zone UTC−12 at 12:00 UTC April 12.
• There are 22 places where three or more time zones meet, for instance at the tri-country border of Finland, Norway and Russia. 28 countries present such triple points, with China being the most present (in 13 of the 22 triple points). Then come India (7), Russia, India and Afghanistan (4).
• There are 40 time zones. This is due to fractional hour offsets and zones with offsets larger than 12 hours near the International Date Line as well as one unofficial zone in Australia. See the list of time zones.
• The largest time gap along a political border is the 3.5 hour gap along the border of China (UTC+08) and Afghanistan (UTC+04:30).
• One of the most unusual time zones is the Australian Central Western Time zone (CWST), which is a small strip of Western Australia from the border of South Australia west to 125.5°E, just before Caiguna. It is 8¾ hours ahead of UTC (UTC+08:45) and covers an area of about 35,000 km², larger than Belgium, but has a population of about 200. Although unofficial, it is universally respected in the area—without it, the time gap in standard time at 129°E (the WA/SA border) would be 1.5 hours. See Time in Australia.

Internet and computer systems

UTC is often used on the Internet for meetings (e.g. IRC chats, news, shows and so on).^[15] For e-mail, the sender time zone is used to calculate the send time, but this time is recalculated by the receiver mail client, and shown according to the receiver time zone.

On websites with mainly domestic audiences local time is often used, sometimes with UTC in brackets: e.g. the international English-language version of CNN includes GMT and Hong Kong Time,^[16] whilst the US version shows Eastern Time.^[17] US Eastern Time and Pacific Time are also used fairly commonly on many US-based English-language websites with global readership.

The format is based in the W3C Note «datetime».

On the other hand, most modern computer operating systems include information about time zones, including the capability to automatically change the local time when daylight saving starts and finishes (see the article on daylight saving time for more details on this aspect).

Operating systems

Unix

Most Unix-like systems, including Linux and Mac OS X, keep system time as UTC (Coordinated Universal Time). Rather than having a single time zone set for the whole computer, timezone offsets can vary for different processes. Standard library routines are used to calculate the local time based on the current timezone, normally supplied to processes through the TZ environment variable. This allows users in multiple timezones to use the same computer, with their respective local times displayed correctly to each user. Timezone information is most commonly stored in a timezone database known as tz database (or sometimes zoneinfo or Olson format). In fact, many systems, including anything using the GNU C Library, can make use of this database.

Microsoft Windows

Windows-based computer systems normally keep system time as local time in a particular time zone. A system database of timezone information includes the offset from UTC and rules that indicate the start and end dates for daylight saving in each zone. Application software is able to calculate the time in various zones. Terminal Servers allow remote computers to redirect their time zone settings to the Terminal Server so that users see the correct time for their time zone in their desktop/application sessions. Terminal Services uses the server base time on the Terminal Server and the client time zone information to calculate the time in the session.

Programming languages

Java

While most application software will use the underlying operating system for timezone information, the Java Platform, from version 1.3.1, has maintained its own timezone database. This database will need to be updated whenever timezone rules change. Sun provides an updater tool for this purpose.^[18]

As an alternative to the timezone information bundled with the Java Platform, programmers may choose to use the Joda-Time library.^[19] This library includes its own timezone data based on the frequently updated tz database.^[20]

JavaScript

There is very little in the way of timezone support for JavaScript. Essentially the programmer has to extract the UTC offset by instantiating a time object, getting a GMT time from it, and differencing the two. This does not provide a solution for daylight savings variations.

PHP

The DateTime objects and related functions have been compiled into the PHP core since 5.2. This includes the ability to get and set the default script timezone, and DateTime is aware of its own timezone internally. PHP.net provides extensive documentation on this.^[21] As noted there, the most current timezone database can be implemented via the PECL timezonedb.

Python

The standard module datetime stores and operates on the timezone information class tzinfo. The third party pytz module provides access to the full tz database.^[22] Negated time zone offset in seconds is stored time.timezone and time.altzone attributes.

Smalltalk

Each Smalltalk dialect comes with its own built-in classes for dates, times and timestamps, only a few of which implement the DateAndTime and Duration classes as specified by the ANSI Smalltalk Standard. VisualWorks provides a TimeZone class that supports up to 2 annually recurring offset transitions, which are assumed to apply to all years (same behavior as Windows time zones). Squeak provides a Timezone class that does not support any offset transitions. Dolphin Smalltalk does not support time zones at all.

For full support of the tz database (zoneinfo) in a Smalltalk application (including support for any number of annually recurring offset transitions, and support for different intra-year offset transition rules in different years) the third-party, open-source, ANSI-Smalltalk-compliant Chronos Date/Time Library is available for use with any of the following Smalltalk dialects: VisualWorks, Squeak or Dolphin.

Databases

Some databases allow storage of a datetime type having time zone information. The SQL standard defines two standard time data types:

TIMESTAMP WITH TIME ZONE

TIMESTAMP WITHOUT TIME ZONE

However, the standard has a somewhat naive understanding of time zones. It generally assumes a time zone can be specified by a simple offset from GMT. This causes problems when trying to do arithmetic on dates which span daylight saving time transitions or which span political changes in time zone rules.

Oracle

Oracle Database is configured with a database time zone, and connecting clients are configured with session time zones. Oracle Database uses two data types to store time zone information:

TIMESTAMP WITH TIME ZONE 

Stores date and time information with the offset from UTC

TIMESTAMP WITH LOCAL TIME ZONE 

Stores date and time information with respect to the dbtimezone (which cannot be changed so long as there is a column in the db of this type), automatically adjusting the date and time from the stored time zone to the client’s session time zone.

PostgreSQL

PostgreSQL uses the standard SQL data types but tries to impose an interpretation which avoids the problems described above.

TIMESTAMP WITH TIME ZONE 

Stores date and time in UTC and converts to the client’s local time zone (which could be different for each client) for display purposes.

TIMESTAMP WITHOUT TIME ZONE 

Stores date and time without any conversion on input or output.

Applications

Microsoft Outlook

Microsoft Outlook has a much-criticized^{[citation needed]} behavior regarding time zone handling. Appointments stored in Outlook move when the computer changes time zone, since they are assumed to be fixed against UTC not against the hour number. As a consequence, someone who inserts an appointment requiring a travel into another timezone will not get a correct time for the appointment after travelling to the other time zone. For example, a New Yorker plans to meet someone in Los Angeles at 9:00 AM. He inserts an appointment at 9:00 AM in Outlook while his computer is on New York time. He travels to Los Angeles and adjusts his computer time zone, which causes the meeting to show up at 6:00 AM (9:00 New York time) in Outlook. One workaround is to adjust the clock but not the timezone of the computer when travelling. This will give sent e-mail wrong time stamp, and new meeting invitations will be wrong. Microsoft recommends^[23] to not change the clock at all and show a second time scale in the calendar. This will give reminder popups at the wrong time, since the clock does not match local time. The Outlook functionality will give correct time if the organizer invites the guest to a meeting using the «invite attendees» feature (the Los Angeles meeting will show up as 12:00 noon in the New Yorkers calender, before he adjusted the time zone), but only if the time zone is adjusted when travelling. The Outlook functionality will also give correct time for telephone meetings. For Outlook 2010 a new feature has been added, the possibility to specify which time zone an event occurs in. This solves most of these problems if properly used. An appointment at 9:00 AM Los Angeles time will show up as 12 AM but at 9 AM on the secondary scale if used.

Time zones in outer space

Orbiting spacecraft typically experience many sunrises and sunsets in a 24-hour period, or in the case of Apollo program astronauts travelling to the moon, none. Thus it is not possible to calibrate time zones with respect to the sun, and still respect a 24-hour sleep/wake cycle. A common practice for space exploration is to use the Earth-based time zone of the launch site or mission control. This keeps the sleeping cycles of the crew and controllers in sync. The International Space Station normally uses Coordinated Universal Time (UTC).

Timekeeping on Mars can be more complex, since the planet has a solar day of approximately 24 hours and 39 minutes, known as a sol. Earth controllers for some Mars missions have synchronized their sleep/wake cycles with the Martian day, because solar-powered rover activity on the surface was tied to periods of light and dark. The difference in day length caused the sleep/wake cycles to slowly drift with respect to the day/night cycles on Earth, repeating approximately once every 36 days.

See also

• Daylight saving time
• ISO 8601
• Jet lag
• List of time zone abbreviations
• List of time zones by country
• Lists of time zones
• Metric time
• Sunpendulum
• Time by country
• World clock

References

External links

• ITU LEGAL TIME 2011
• W3C Note Datetime
• US Official Time Clock Java-enabled clock to graphically display night and day around the globe.
• PlanetaryTime Local times world wide & time zone details.
• Time Zone Map Current times worldwide.

Often, we hear the term «Time Zone» when it’s used to describe the current time for different areas of the world.

What it actually refers to is one of the specific regions or areas — out of the 24 total regions in the world — that are divided up by longitude. Within each one of those regions, a standard version of time is maintained.

The different time zones are calculated based on their relation to the coordinated universal time or UTC.

 Related: What is UTC?

How Are Time Zones Determined? 

What you may know as the prime meridian — or more formally the Greenwich Meridian — is the initial division line for all time zones. It’s where dividing the Earth up into regions effectively starts.

The prime meridian is an imaginary line — invisible by all rights — that runs from north to south along the 0 degree longitude line of Earth. It is more commonly associated with Greenwich, England where the line passes through.

Each of Earth’s 24 zones is one hour earlier then the one east of it, which equates to about 15 degrees longitude for each. Although this is the general rule, there’s not always a one hour difference between each time zone.

Some countries have adopted a non-standard time, which may include a 30 or 45 minute offset. For instance, one of the more popular forms of this is in India. The India time zone is actually five hours and 30 minutes earlier than UTC time (UTC +5:30).

There are many reasons why this might be true. Sometimes it has to do with political factors, like in India, but other times it might have to do with the location of a countries’ borders. Since time zones are defined by their longitude coordinates, some countries even have multiple time zones within them. Russia and the United States have multiple time zones within their borders, whereas other countries like India and China only use one.

Why Do Time Zones Exist? 

Before the late 19th century, the standard for clocks was usually discerned by the sun and star patterns. It’s just one of the reasons why astronomy was so important back then. This makes a lot of sense too considering the sun rises (dawn) and sets (dusk) at different times all over the world.

At the same time, travel back then — which was primarily by sea — took long periods of time. That meant that when people crossed over between time zones, they didn’t experience much of a difference in terms of the local time.

However, when transportation and global communications became much more prominent in the 19th century, a need for a more unifed and universal time standard arose.

Naturally, in 1884 the Greenwich Meridian was designated as the central or «prime» meridian for the measurement of longitude and timekeeping.

How Is Standard Time Decided?

Each time zone is 15 degrees wide and there’s a one hour difference between each one. Depending on the distance — east or west — from the Greenwich Meridian you must either add or subtract the appropriate time for every 15 degree interval.

To find the time zone in hours of a particular location, you can take the longitude — in degrees — and divide it by 15. So, for example, 75° E would be 75/15 which equals 5. That translates to the time zone being 5 hours ahead of UTC or GMT time, which can also be labeled as UTC+5.

Let’s do another one. 30° E would be 30/15 which equals 2. When you match that with the correct time zone description it’s UTC+2.

Obviously, if the latitude is west then you would subtract the appropriate hours. 150° W is UTC-10, because 150/15 equals 10.

At timeanddate.com we define a time zone as a region where the same standard time is used.

Time Difference from UTC

The local time within a time zone is defined by its offset (difference) from Coordinated Universal Time (UTC), the world’s time standard.

UTC time changes 1 hour forward and backward corresponding to a 1-hour difference in mean solar time for every 15 degrees east or west of the prime meridian (0° longitude) in Greenwich, London, United Kingdom. The offset is expressed as either UTC- or UTC+ and the number of hours and minutes.

Interactive Time Zone Map

More Than 24 Time Zones

If each time zone were 1 hour apart, there would be 24 in the world. However, the actual borders on the time zone map have been drawn to match up with both internal and international borders, and rarely match up exactly with the 15-degree longitudes. Also, the International Date Line (IDL), creates 3 time zones and several time zones are only 30 and 45 minutes apart. This makes the total number of time zones worldwide much higher.

Daylight Saving Time Zones

Regions that use Daylight Saving Time (DST) change the time zone name and time during the DST period. The words “daylight” or “summer” are then usually included in the time zone name. The areas that don’t use DST remain on standard time zone all year.

For example, California uses Pacific Daylight Time (PDT) during the DST period, but Pacific Standard Time (PST) during the rest of the year.

Not the Same as Local Time

The term time zone is often used instead of local time. For instance, during DST, it is common to say “California and Arizona are now in the same time zone.” However, the correct thing to say would be: “California and Arizona now have the same local time.”

The reason is that California’s local time during DST is UTC-7, but the standard time in California is minus one more hour: UTC-8. However, Arizona’s local time is always UTC-7, because there’s no DST in Arizona, and they remain on standard time all year.

Why do we have time zones?

DST worldwide

Local Time Zone Names

To confuse matters more, each time zone can have different local time zone names, usually linked to the geographical name of the country or region. The time zone names may be completely different, even though the UTC offset is the same.

For instance in Miami, Florida, is 5 hours behind UTC (UTC-5) and the standard time zone is Eastern Standard Time (EST). In Havana, Cuba, the standard time zone is also UTC-5, but it’s called Cuba Standard Time (CST).

Military Time Zones

There are also 25 military time zones which follow the rule of 1 hour per 15 degrees longitude. These are named according to the NATO phonetic alphabet: Alpha, Bravo, Charlie, etc. and are used in aviation, at sea, and in telecommunications.

The reason there are 25 military time zones instead of 24 is that Mike Time Zone (M) and Yankee Time Zone (Y) are the same time, but on either side of the International Date Line. J (Juliet Time Zone) is occasionally used to refer to the observer’s local time.

Learn how to use military time

Identical Abbreviations

Another point that can cause confusion is that some time zone’s names in totally different places have exactly the same abbreviation. For example, India Standard Time (IST) and Israel Standard Time (IST) have the same abbreviation, but completely different UTC offsets of UTC+5:30 and UTC+2:00.

In many parts of the world, especially in countries with only one time zone, time zone names are not commonly used at all.

Time zone names worldwide

Hot Political Potato

In most countries, the political decision to make adjustments regarding time zones or DST is made for practical reasons, like saving energy, facilitating trade with neighboring areas, or boost tourism.

In some cases, time zone borders and DST can be a political tool, most recently in Russia, Ukraine, and North Korea.

Topics: Time Zone, Timekeeping

We explain what time zones are and why they are necessary. And as an example, list of time zones around the planet.

1. What is a time zone?

The time zone is a concept that comes from geography, and that consists of each of the twenty-four regions of chronometric time in which our planet is divided . Each of these regions is called the “time zone” or “time slot” and has a specific assigned time, which applies to all populations whose geographic location shares the same meridian.

Said in simpler terms, it is a division of the planet Earth in spindles (strips, columns) that allows us to calculate the time in each country of the world , adding or subtracting one hour (1 h) according to the amount of time zones that separate us from him. For example, if there are three time slots between a country X and a country Y, it will be necessary to add or subtract three hours while registering the clocks of one, to determine the exact time of the other.

The system arises because the Earth rotates on its axis from west to east, causing the Sun to illuminate a portion while leaving the other in the dark, and a method is necessary to standardize time . Thus arose the Coordinated Universal Time (UTC), corresponding to the “zero meridian”: the Royal Greenwich Observatory in England, United Kingdom (UK).

Said “universal” time is accepted by convention and serves to determine the others, adding or subtracting hours as they are, respectively, further east or west of this time zone (0). In the same way, the 180 ° meridian is considered in this system the international date change line , that is, the point at which the day ends and a new one is born.

The time zones are a guide, a reference for time management in a world in which we can move quickly through its surface. Each nation on the planet has chosen the time zone to govern , often determined by the one corresponding to its capital.

Some particularly voluminous nations handle more than one time zone, such as the United States, Russia, Australia or Canada, as they are crossed by several meridians. In other cases, nations alternate between one and another time zone, according to the weather station in which they are located, to make the most of the limited or abundant hours of daylight, and thus save electricity .

Time zones are traditionally denoted by the acronym UTC and the corresponding addition or subtraction of the hours of separation from the zero meridian.

1. Examples of time zones

Below is a partial list of time zones and the nations that choose them:

• UTC – 12 . Howland and Baker Islands (United States).
• UTC – 11 . American Samoa.
• UTC – 10 . Hawaii, French Polynesia, Cook Islands (New Zealand).
• UTC – 9:30 . Marquesas Islands (French Polynesia). 
• UTC – 9 . Continental Alaska, Gambier Islands (French Polynesia).
• UTC – 8 . Canada, United States (west), Mexico.
• UTC – 7 . Canada, United States (west-central), Mexico.
• UTC – 6 . Belize, Canada, Chile, Costa Rica, Ecuador (insular), El Salvador, United States (center-east), Guatemala, Honduras, Mexico, Nicaragua.
• UTC – 5 . Bahamas, Brazil, Canada, Colombia, Cuba, Ecuador (continental), United States (east), Haiti, Jamaica, Panama, Peru, Cayman Islands.
• UTC – 4 . Antigua and Barbuda, Barbados, Bolivia, Brazil, Canada, Dominican Republic, Dominica, Greenland, Grenada, Guyana, the Netherlands Antilles, Paraguay, Puerto Rico, Trinidad and Tobago, Venezuela.
• UTC – 3 . Argentina, Brazil, Chilean Antarctica, Greenland, French Guyana, Falkland Islands, Suriname, Uruguay.
• UTC – 2 . Brazilian coast, Sandwich Islands (United Kingdom).
• UTC – 1 . Greenland, Cape Verde, Azores Islands (Portugal).
• UTC 0 . Burkina Faso, Ivory Coast, Canary Islands (Spain), Ghana, Ireland, Iceland, Liberia, Morocco, Portugal, United Kingdom (England, Scotland, Wales, Northern Ireland), Senegal, Togo.
• UTC + 1 . Germany, Albania, Bosnia and Herzegovina, Belgium, Cameroon, Austria, Spain, Slovenia, Denmark, France, Hungary, Macedonia, Italy, Nigeria, Norway, Netherlands, Czech Republic, Vatican City, Tunisia, Switzerland, Sweden.
• UTC + 2 . Bulgaria, Botswana, Jordan, Israel, Finland, Egypt, Estonia, Palestine, Namibia, Libya, Rwanda, Romania, Syria, South Africa, Ukraine, Zimbabwe.
• UTC + 3 . Saudi Arabia, Belarus, Qatar, Ethiopia, Iraq, Kenya, Kuwait, Madagascar, Russia (west), South Sudan, Turkey, Yemen.
• UTC + 3:30 . Iran.
• UTC + 4 . Armenia, United Arab Emirates, Russia (center-west), Oman.
• UTC + 4:30 . Afghanistan.
• UTC + 5 . Kazakhstan, Pakistan, Maldives, Russia (center-east).
• UTC + 5:30 . India, Sri Lanka.
• UTC + 5:45 . Nepal.
• UTC + 6 . Bangladesh, Kyrgyzstan, Russia (Omsk only).
• UTC + 6:30 . Cocos Islands (Australia) and Burma.
• UTC + 7 . Cambodia, Indonesia, Laos, Mongolia, Russia (east), Thailand, Vietnam.
• UTC + 8 . Australia (west), Brunei, China, island Indonesia, Malaysia, Philippines, Singapore, Russia (far east).
• UTC + 8:30 . North Korea.
• UTC + 9 . South Korea, Japan, Palau, East Timor, Russia (yes, it is huge).
• UTC + 9:30 . Australia (center).
• UTC + 10 . Australia (east), Micronesia, Russia (insular region).
• UTC + 11 . Vanuatu, Papua New Guinea, Solomon Islands, Russia (Far East).
• UTC + 12 . Fiji, Nauru, New Zealand and the latest from Russia.
• UTC + 13 . Tonga, Tokelau, Samoa.
• UTC + 14 . Kiribati

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WTO is not the forum for labour standards. Next, the U.S. will argue the time zone difference is an unfair competitive advantage enjoyed by India that enables our software engineers to work while the Americans sleep.

Jairam Ramesh

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REFERENCE

« EDUCALINGO. Time zone [online]. Available <https://educalingo.com/en/dic-en/time-zone>. Apr 2023 ».