Using the apostrophe can cause the best of us to quail. One aspect of apostrophe use which seems to cause particular trouble is using apostrophes with times, such as in one year’s time or she has seven years’ experience.
Something I see quite often is confusion on when you need to use an apostrophe with times and when you don’t.
You use an apostrophe with a time when there’s a possessive element: i.e. in two years’ time means the time belonging to two years. You don’t need an apostrophe when the time is an adjective, and when it’s describing something.
How can I tell the difference?
Once you know how to tell what’s a possessive and what’s an adjective, this should make life much simpler. If you can add the word of to the sentence, then it’s a possessive, and it needs an apostrophe. If you add of to the sentence and it sounds nonsensical, leave the apostrophe out.
She has seven years’ experience = She has seven years of experience
In one year’s time I will be 70 = In one year of time I will be 70
This makes complete (if slightly awkward) sense. It’s therefore correct to use an apostrophe.
She is eight months pregnant
Neither of these examples needs an apostrophe. Let’s try adding an of to check:
She is eight months pregnant = she is eight months of pregnant
Out-of-hours calls = out-of-hours of calls
Those are both things you wouldn’t ever say, so you don’t need an apostrophe.
Apostrophes when used with time follow exactly the same rules as when you use apostrophes with possessives. If there’s just one month, or hour, or year, the apostrophe goes before the ‘s’. If you’re talking about months, hours or years, the apostrophe goes after the ‘s’.
In one year’s time we shall finish the course.
In one minute’s time I shall be ready to go.
In two years’ time we shall be ready to start another course.
In 10 minutes’ time I shall be ready to leave.