two months' rent, two months' notice, two months' salary and two months' living expenses

Question

(1a) I lost my job. However, I have enough money for two months' living expenses.

(1b) I lost my job. However, I have enough money for two months' of living expenses.

(2a) Please give me two months' notice before you move out.

(2b) Please give me two months of notice before you move out.

(3a) Jack owes two months' rent.

(3b) Jack owes two months of rent.

(4a) I spent two months' salary on my new car.

(4b) I spent two months of salary on my new car.

Some of my friends think it's correct to say "two months' of living expenses". For the rest of my sentences, you say "two months' notice/rent/salary. I don't understand why. Please explain that.

Answer 1

In "two months' rent", the apostrophe indicates a possessive: it is equivalent to "two months of rent". Because months is plural, the apostrophe goes after the s. If you use singular, the apostrophe goes before the s, for example "a week's rent".

This format is mostly used for time, but can be used for other quantities:

a dollar's worth
ten dollars' worth

Note that you cannot use the possessive form when it is preceded by a determiner, or when it is not followed by a noun:

It is a five minutes' walk - incorrect
It is five minutes away - correct

You can indicate possession using an apostrophe or of: you must not use both. Compare these phrases:

the soldiers' swords - correct
The swords of the soldiers - correct
The swords' of the soldiers - incorrect

Your friends' suggested phrase is incorrect, because you must not use both an apostrophe and an of.

two months' of living expenses - incorrect