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Do yesterday and one day ago refer to the same time period? If no, what is the difference?

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up vote 4 down vote accepted

There is no real difference that would be recognized and respected by all writers and readers.

However, I can see some justification in separating the two. If it is 6 o’clock in the morning, then yesterday could refer to any time before you went to bed for the night during the previous day, including for example 6 at night.

On the other hand, a day ago would be more apt to refer to a time close to exactly 24 hours earlier, so 6 in the morning again.

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That's exactly what I thought. Thank you – 271 Dec 25 '12 at 19:33
They're both Deictic terms, which are explained in the Deixis Lectures. In this case, probably Lecture 2 'Time'. – John Lawler Dec 25 '12 at 19:40
@JohnLawler thank you for the links, it's very helpful – 271 Dec 25 '12 at 19:44
You're welcome. I think everybody who claims to be interested in language or in the English language should be familiar with those lectures. Information that is very clearly explained in them is the source of the answers for a significant fraction of questions asked here. – John Lawler Dec 25 '12 at 19:47
Yesterday refers to calendar date, so there can be a difference if a reference crosses time zones. It's correct to say, if you're in England at half past midnight, "It's still yesterday in the United States." – gmcgath Dec 25 '12 at 21:32

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