Apostrophes in expressions of time

Question:

In the following example, does years take an apostrophe?

• The Agency plans to assess changes in three years' time.

Answer:

The apostrophe is one of the most commonly used and misused punctuation marks. To help set the record straight, we consulted some reliable Canadian sources.

The Canadian Style indicates that certain expressions of time and measurement take the apostrophe. For example:

• She plans to take one month's vacation.

So, in your example, the word years does take an apostrophe:

• … in three years' time.

The Gregg Reference Manual agrees, adding that plural nouns are often possessive if followed immediately by another noun (e.g. two weeks' vacation). To decide if the possessive form should be used, try using an of phrase instead (e.g. a vacation of two weeks). If this substitution works, then the possessive form is correct.

And The Little, Brown Handbook recommends adding an apostrophe to form the possessive case of plural nouns ending in -s (e.g. several years' work).