Is it correct to use possessive case for referring to the time in consideration, like in
in today's US
in modern's US
in last century's England
closed as general reference by MετάEd, tchrist, Mitch, coleopterist, Cameron Oct 16 '12 at 5:21
This question is too basic; it can be definitively and permanently answered by a single link to a standard internet reference source designed specifically to find that type of information. If this question can be reworded to fit the rules in the help center, please edit the question.
Possessives in English can generally only be used to modify nouns (strictly speaking, noun phrases). In your examples, two are nouns but one is not: