OT, but noting the variation in form here I got curious about the usage of apostrophes to designate groups of years. Style guides allow several forms:
For groups of years, the apostrophe at the end may be regarded as unnecessary, since there is no possibility of misreading. For this reason, some style guides prefer 1960s to 1960's (although the latter is noted by at least one source as acceptable in American usage), and 90s or '90s to 90's or '90's.
http://en.wikipedia.org/wiki/Apostrophe
Looks like everyone is "correct"