Word meaning by this time

Dear All,

By this time tomorrow, I’ll be lying on the beach.

Above sentence is from my book, I don’t know why there is a by in front of the time phrase. Usually, no preposition is needed when we predict what will be going on at the same time in the future. For example, ‘This time tomorrow, I’ll be sleeping/having breakfast.’

Could you please give me some suggestion on it?

Bob

«This time tomorrow» means «at this precise time tomorrow».

«By this time tomorrow» means «no later than this time tomorrow» or «before this time tomorrow».

Can I say, ‘By this time tomorrow, I’ll be lying on the beach.’ = ‘This time tomorrow, I’ll already be lying on the beach.’ ?

‘This time tomorrow, I’ll be sleeping/having breakfast.’

It seems to me that the above sentence has a word missing…»by» or «at».

We have gone a bit far. I hope somebody can tell the difference between following two sentences:

At this time tomorrow, I’ll be lying on the beach.

By this time tomorrow, I’ll be lying on the beach.

I learned that the use of «this time» with «yesterday» in past tense sentences as below:

a. this time yesterday (but this is usually used at the beginning of the sentence)
b. yesterday at this time
c. at this time yesterday
d. yesterday this time (It is never acceptable.)

So, I wonder whether this situation applies to «this time tomorrow» in the same way:

a. this time tomorrow (Is this usually used at the beginning of the sentence?)
b. tomorrow at this time (Both beginning and the end of the sentence?)
c. at this time tomorrow ((Both beginning and the end of the sentence?)
d. tomorrow this time (Is it never acceptable?)

Thanks a lot.

By this time tomorrow she will have travelled over 100 km

Coud that example cover two scenarios?
Scenario 1: She’s already started travelling, she is travelling now and will continue tomorrow.
Scenario 2: She hasn’t started travelling yet. She wil start, e.g., tomorrow morning and by this time (say it is noon now) she will have driven over 100km.

Yes it could. The sentence only tells us about the instant she will have covered over 100 km but it does not tell us, or restrict the possibilities, of how or when she did the traveling.

And does the future perfect simple work too and mean the same? If so, do you sense any difference?

1 By this time tomorrow she will have been travelling for two months.
2 By this time tomorrow she will have travelled for two months.

1 conforms to

Right. The action started very nearly two months ago. The start time is indicated by the present continuous — from start to the present.

and the start time is indicated by the present continuous — from start continuing to the present.
2 tells us that in her life the total traveling time will add up to two months. The start time is not implied or specified.

Now you can see how the meaning of the verb is a consideration, not just some grammar rule. Working and playing the piano are both either a state/capability or an action with duration. You need context to sort them out.

Consider:
I’ve been playing the piano for five minutes. Most likely one continuous action.
I’ve been playing the piano for five years — not likely a continuous feat. This is a capability/pastime that does not stop applying when you are asleep and not at the piano (or at work), for example.

Then when you shift it to a future perfect, you add logic to determine what possible meanings the sentences might have.
In both your new examples, I can’t see much possible difference in meaning; can you?