From week number to date excel

If you are looking for the opposite to get a date from a weeknumber i found a solution online and changed it slightly:

Function fnDateFromWeek(iYear As Integer, iWeek As Integer, iWeekDday As Integer)
  ' get the date from a certain day in a certain week in a certain year
  fnDateFromWeek = DateSerial(iYear, 1, (iWeek * 7) _
          + iWeekDday - Weekday(DateSerial(iYear, 1, 1)) + 1)
End Function

I took the formular from asap-utilities.com/ and changed

DateSerial(iYear, 1, ((iWeek - 1) * 7) 

to

DateSerial(iYear, 1, (iWeek * 7)

Update

It seems that at least for germany the formular works not as expected for the leap year 2016 so i changed it to this

Function fnDateFromWeek(iYear As Integer, iWeek As Integer, iWeekDday As Integer)
' get the date from a certain day in a certain week in a certain year
  
    If isLeapYear(iYear) Then
        curDate = DateSerial(iYear, 1, ((iWeek) * 7) _
          + iWeekDday - Weekday(DateSerial(iYear, 1, 1)) + 1)
    Else
        curDate = DateSerial(iYear, 1, ((iWeek - 1) * 7) _
          + iWeekDday - Weekday(DateSerial(iYear, 1, 1)) + 1)
    End If
    fnDateFromWeek = curDate
End Function

Since 2016 hardcoded is not ideal you could check if a year is a leap year with this function

Function isLeapYear(iYear As Integer) As Boolean

    If (Month(DateSerial(iYear, 2, 29)) = 2) Then
        isLeapYear = True
    Else
        isLeapYear = False
    End If
    
End Function

For a non-leap-year DateSerial(iYear,2 ,29) returns 1st of march

This may still be wrong but my limited test gave the expected results:

Sub TestExample()
   Debug.Print Format(fnDateFromWeek(2014, 48, 2), "ddd dd mmm yyyy") ' mo 24 Nov 2014
   Debug.Print Format(fnDateFromWeek(2015, 11, 6), "ddd dd-mmm-yyyy") ' fr 13 Mar 2015
   Debug.Print Format(fnDateFromWeek(2016, 36, 2), "ddd dd-mmm-yyyy") ' Mo 05 Sep 2015
End Sub

This article will explain how to calculate a date from any week number.

ISO Week Number

Before showing you the formulas, it is important to keep in mind that there are 2 ways to count weekly numbers

• The North American method (the formulas WEEKNUM in Excel)
• The international method (the formula ISOWEEKNUM)

In Europe, the calculation of week numbers is governed by the ISO 8601 standard. This standard considers that the first week of the year must have at least 4 days. When the first week of the year begins on a Friday, Saturday or Sunday, these days are not considered the first week of the year.

But for North America, this rule does not exist. The calculation of week numbers starts on January 1st, even if January 1st is a Sunday.

With Excel, this difference can be displayed as follows:

• With the WEEKNUM function, regardless of the first day of the year, the function always returns the value 1.
• With ISOWEEKNUM, if the first day of the year is a Friday, Saturday or Sunday, the function considers that the date of January 1 belongs to the week of the previous year.

This notion is important in the result we are going to achieve

Common formula to return a date according the week number

The common formula to return on the first day of a week (the week begins on a Monday) is as follows:

=DATE(Year,1,1)+(Week-1)*7-WEEKNUM(DATE(Year,1,1),2)+1

If the week starts on a Sunday, simply remove the +1 at the end

=DATE(Year,1,1)+(Week-1)*7-WEEKNUM(DATE(Year,1,1),2)

Mistake made with the formula 

But this formula does not take into account the problem of ISO week numbers. This function works for the United States and Canada, but not for other countries 

To find out, the following table shows the problem

• The value of the years in column A
• The day of the first of January in column B; =DATE(A2,1,1)
• The result of the calculation of the date from the first week
• The ISO week number

In the previous table, we see that the date calculated for the first week is often in December. In fact, the formula returns the month of January only if January 1 is a Monday. 

Now, if we consider the year 2021, we can see that 01/01/2021 is a Friday. And so, the calculated Monday will be 28/12/2020. But in this case, we are no longer in week 1 (ISO week number)

Hence a one-week delay compared to calculation standards in Europe 

Therefore, the previous formula must be adapted to handle the ISO week number rule.

Formula that takes into account the ISO week rule

Taking into account the previous remarks, the formula that allows calculating a date from the week number respecting the iso week rule is as follows:

=DATE(Year,1,1)+IF(WEEKDAY(DATE(Year,1,1),2)<5,(WeekNum-1)*7,WeekNum*7)-WEEKDAY(DATE(Year,1,1),2)+1

As you can see, the calculation of the first Monday of the first week of the year is now correct 😃👍

So now, you can replace the week number by any values between 1 and 52 and the result will be correct for the ISO week number system.

This post will guide you how to convert a week number to a date with a formula in Excel. How do I calculate a date from a week number and a year with formula in Excel. Or how to get week number from a given date with formula in Excel.

Convert Week Number to Date

Assuming that you have a week number and a year number in your worksheet, and you want to get the start date and end date in that give week in that year. How to achieve it.

You need to create a formula based on the MAX function, the DATE function and the WEEKDAY function to convert week number to a date in Excel. Just do the following steps:

#1 Type the year number in Cell C1, and type the week number in Cell C2.

#2 Type this formula into the formula box of the Cell C3, then press Enter key in your keyboard. The Serial number of the start date is calculated in Cell C3.

=MAX(DATE(C1,1,1),DATE(C1,1,1)-WEEKDAY(DATE(C1,1,1),2)+(C2-1)*7+1)

#3 Type the following formula into the formula box of the Cell C4, and then press Enter key in your keyboard. And the serial number of the end date is calculated in Cell C4.

=MIN(DATE(C1+1,1,0),DATE(C1,1,1)-WEEKDAY(DATE(C1,1,1),2)+C2*7)

#4 select the cells C3:C4 and then go to HOME tab, click Number format list box, and select Short Date item. The serial number are changed to a standard date format.  Or you can also select Long Date format from the drop-down list of the Number format.

Get Week Number From a Date

If you want to get the week number from a given date, then you can use the WEEKNUM function to quickly achieve it. Just type the formula into the Cell D3:

=WEEKNUM(C3,1)

• Excel MIN function
  The Excel MIN function returns the smallest numeric value from the numbers that you provided. Or returns the smallest value in the array.The MIN function is a build-in function in Microsoft Excel and it is categorized as a Statistical Function.The syntax of the MIN function is as below:= MIN(num1,[num2,…numn])….
• Excel MAX function
  The Excel MAX function returns the largest numeric value from the numbers that you provided. Or returns the largest value in the array.= MAX(num1,[num2,…numn])…
• Excel DATE function
  The Excel DATE function returns the serial number for a date.The syntax of the DATE function is as below:= DATE (year, month, day)…
• Excel WEEKDAY function
  The Excel WEEKDAY function returns a integer value representing the day fo the week for a given Excel date and the value is range from 1 to 7.The syntax of the WEEKDAY function is as below:=WEEKDAY (serial_number,[return_type])…
• Excel WEEKNUM function
  The Excel WEEKNUM function returns the week number of a specific date, and the returned value is ranging from 1 to 53.The syntax of the WEEKNUM function is as below:=WEEKNUM (serial_number,[return_type])…