Apostrophes in expressions like 2 years' pay and a day's notice

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 single units of time and after for multiple units of time. For example:

• I never did a day's work in my life. It was all fun.  (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.)

These are both correct.
(newspaper clipping)


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

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