Time Expressions with “in”

Question

I have seen some examples about using "in" with time expressions. They give some examples as shown below:

I will leave for vacation in a month. (Exactly one month from now I will go on vacation.)

We’ll be there in an hour. (Exactly one hour will go by and then we will arrive.)

But these comments confused me. Then I looked the usage of "in" up in a dictionary and it says about usage of "in" with time expressions :"before or at the end of a particular period". I think the comments and the definition in the dictionary contradict each other?

dictionary

I have examples of mine:

1- Dinner will be ready in ten minutes. (Maybe dinner will be ready at the 5th minute? or 8th. Does it strictly mean dinner will be ready at the end of the ten minutes?)

2- My letter arrived in two days. ( Does that mean it took exactly two days to arrive? or Could it be just one day because he/she doesnt remember the exact time?)

3- Imagine you are on the first day of a month and your friend says: I will be going outside the country in this month. (Does that mean he/she could go tomorrow or ten days later or at the end of the month?)

4- He can run a mile in 5 minutes. ( Does that mean he has to spent entire 5 minutes to run a mile? Could it be 4 or 4.5 minutes?)

Answer 1

Like many other things in English, before or at the end of a particular period can be ambiguous. As with words that have multiple meanings, you can only tell what is actually meant in any given situation by its context.

For instance:

I will be there in an hour.

This normally means to expect the person to arrive at the end of an hour—although they could still show up earlier than that.

However:

I will be there sometime in the next hour.

Here, additional wording indicates that there is no set time—except for expecting the person to arrive no later than an hour from now. The use of sometime gives it the sense as within. But even if sometime isn't used, the next also conveys that as a subtle suggestion—more than just an does.

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As the dictionary definition says, the word in can be used in both contexts.

Generally speaking, I'd say it's more common for in to mean at the end of the time mentioned than for it to mean within the time period mentioned. If you assume the first interpretation, you'll be right more often than not.

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The conversational use of in is mostly understood, but it's still open to misinterpretation.

If somebody wants to deliberately convey an explicit piece of information about timing, then they would phrase it differently:

I will be there in exactly an hour.
I will be there at 2:00 p.m.