The 13 April 2010 issue of the NYT "After Deadline FAQs on Style" blog stated: [...] But do not use apostrophes for plurals of abbreviations without periods, or for plurals formed from figures [...]" and gave as an example "1990s". The exception would be when you want to indicate the possessive form of a date, such as, "2015's temperatures were warmer than those of the previous year." An apostrophe may also be used in a contraction of a date, such as, "the '90s were an interesting decade". The Times' stylebook use of the apostrophe in a non-possessive date does hold when typesetting an all-caps headline, as in, "PUNCTUATION IN THE 1990'S" in order to keep the date from looking like the model number of a European car ("the new 1990S"). In other words, 1990's is not normally correct, it's a catapostrophe (yeah, real word).
Re: “Mount Hope Cemetery: A buried treasure”
The 13 April 2010 issue of the NYT "After Deadline FAQs on Style" blog stated: [...] But do not use apostrophes for plurals of abbreviations without periods, or for plurals formed from figures [...]" and gave as an example "1990s". The exception would be when you want to indicate the possessive form of a date, such as, "2015's temperatures were warmer than those of the previous year." An apostrophe may also be used in a contraction of a date, such as, "the '90s were an interesting decade". The Times' stylebook use of the apostrophe in a non-possessive date does hold when typesetting an all-caps headline, as in, "PUNCTUATION IN THE 1990'S" in order to keep the date from looking like the model number of a European car ("the new 1990S"). In other words, 1990's is not normally correct, it's a catapostrophe (yeah, real word).