Reply #24: often seen used with decades

[fishwax]

as in "the conformity of the 1950's gave way to the social activism of the 1960's."

(Personally, I think this is acceptable sometimes, when implying posession ... some people in my town have a problem with a business called the "classic 50's diner," but I always argue they're saying the diner "belongs to" the 50s in a metaphorical sense, so I don't mind that one particularly. But usually it's just wrong.)