We use an apostrophe -s construction (in a year’s time, in two months’ time) to say when something will happen.
Sometimes, I see people use "5 days' leave" or "On New Year’s Day"
Why not "5 day leave" or "On New Year Day" or "in a year time"?
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The question addresses Inanimate Possessives.
As a rule, nouns referring to inanimate things should not be in the possessive. Use an “of” phrase instead.
the bottom of the barrel (NOT: the barrel’s bottom)
- the wording of the agreement (NOT: the agreement’s wording
- the lower level of the terminal (NOT: the terminal’s lower level)
However, in reference to time and measurement, and in phrases implying personification, possessive form has become accepted usage:
- a day’s notice
- an hour’s work
- two years’ progress (plural possessive)
- two weeks’ salary (plural possessive)
Time periods are sometimes put into possessive form, to express the duration of or time associated with the modified noun:
- the Hundred Years' War
- a day's pay
- two weeks' notice
The paraphrase with of is often un-idiomatic or ambiguous in these cases.
's means "of".
So "five days' leave" means "a leave of five days". "New Year's Day" means "the day of the new year". With such named days, usage has varied so the apostrophe is rarely used.
An alternate for the former case is to omit the "of" and make the duration an adjective by adding a hyphen: A five-day leave.