Explanation:
While it is true that whatever noun attracts apostrophe (possession) must be an animate, it is also true of time, or for nouns depicting time- minute, hour, day, month and year.

See examples: she has two months' holiday every year. They only gave us two weeks' notice to evacuate the place.
I will see you in an hour's time
The tenant owed several month rent.
In the above examples, words with apostrophe are genitive nouns.