Apostrophes and Years

I had never really paid much attention to it before, but I've noticed that sometimes people will write 1600's or 1970's and include an apostrophe while other times it would be omitted (1970s).  Thinking about it logically it doesn't really make sense to have an apostrophe since it usually denotes possession.  In the sentence "The 1970's were a great decade.", neither the year 1970 nor the decade is owning anything.  Interestingly the Chrome spellchecker is flagging the 1970s as an error and the version with an apostrophe as ok.  Changing the setting between US English and British English doesn't seem to make a difference.

After doing some reading it appears that the answer is not completely straightforward.  It seems that in general it's a matter of style.  So if you decide to use an apostrophe then be consistent and use it all the time.  The same goes for if you decide not to.  It also seems that the apostrophe may be more prevalent in American English than British English.  I found some mentions that there is a convention to use an apostrophe for plural acronyms and words that don't have a natural plural, though I'm not completely convinced of this.

It seems like there could be some ambiguity with using an apostrophe.  For example "1970's hottest month was July."  In this case the individual year 1970 does own a hottest month, and this does not refer to the decade.  I suppose you would be able to figure it out from the context though.  Overall the message seems to be do what feels right and stick to your choice.