Using Apostrophes in Time Expressions (Temporal Expressions)

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The Quick Answer
Apostrophes are used in time expressions. For example:
  • 1 year's insurance
  • 2 days' leave
  • a week's pay
When it is one measure of time (e.g., a day, one week), the apostrophe goes before the "s" (e.g., a day's pay, one week's vacation). When it is more than one measure of time (two days, five weeks), it goes after the "s" (e.g., two days' pay, five weeks' vacation).
apostrophes in time (temporal) expressions

Apostrophes in Time Expressions (Temporal Expressions)

Apostrophes are used in time expressions (e.g., three years' experience, two days' pay, one day's time). These are also known as "temporal expressions."

In a temporal expression, the apostrophe is positioned before the "s" for a single unit of time (e.g., a day) and after for plural unit of time (e.g., two days). For example:
  • I never did a day's work in my life. It was all fun. (Inventor Thomas A. Edison)
  • (As this is "one day," the apostrophe goes before the "s.")
  • Alan was given two days' notice.
  • (As this is "two days," the apostrophe goes after the "s.")
  • That is the equivalent of one year's pay.
  • (As this is "one year," the apostrophe goes before the "s.")
  • My car came with three year's free insurance.
  • (As this is "three years," the apostrophe should go after the "s.")

Imagine the Apostrophe Replaces "Of"

The following do not have any apostrophes in them:
  • I lived in Africa for 3 years.
  • She has six months left to run on her loan.
This point causes confusion among many. As a rule, you should only use an apostrophe in an expression where the word "of" might have been used. For example:
  • six months' insurance
  • (six months of insurance)
  • a day's leave
  • (a day of leave)
  • She has six months' left to run on her loan.
  • (She has six months of left to run on her loan.)
    (This is nonsense. It's wrong.)
  • She has six months left to run on her loan.
  • (This is correct with no apostrophe.)

It's Not Always about Time

The vast majority of these expressions are time expressions, but some relate to value and distance too:
  • 10 pounds' worth of potatoes and 1 pound's worth of onions
  • a stone's throw away

Real-life Examples of Temporal Expressions

Here are some extracts from newspapers:
 
(These are both correct.)

 
(This is wrong. It should be "3 months' mobile insurance.")

 
(This is wrong. It should be "4 years' free credit.")

Learn about Apostrophe Placement in Temporal Expressions

Play around with this widget to learn more about apostrophe placement with temporal expressions:

This widget is in Learning Mode.
Possessor
Possessee
Singular Getting ready... Getting ready...
Plural Getting ready...
Getting ready...
Interactive Exercise
Here are three randomly selected questions from a larger exercise, which can be edited, printed to create an exercise worksheet, or sent via email to friends or students.

See Also

Using apostrophes The apostrophe error with plurals Apostrophes replace letters Apostrophes to show the plural of abbreviations Apostrophes show possession