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Abbreviated date-ranges like "’90s" are contractions, so the apostrophe goes before the numbers.

You can also completely omit the apostrophe if you want: "The 90s were a bit weird."

Numeric date-ranges like 1890s are treated like standard nouns, so they shouldn't include apostrophes.

To show possession, the apostrophe should go after the S: "That was the ’90s’ best invention."

The apostrophe should only precede the S if a specific year is being discussed: "It was 1990's hottest month."

TL;DR: When writing dates, apostrophes do not pluralize!

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