[found on grammarphobia.com; by The Grammarphobia Blog]

 Nouns: Possessive vs Genitive

“Normally, nouns used with numbers to form adjectival phrases are singular, as in “two-inch rain,” “three-year-old boy,” “two-dollar word,” “eight-volume biography,” and “four-star restaurant.”

However, where a plural noun is used by tradition to form such a phrase, it’s generally followed by an apostrophe, as in “the Thirty Years’ War” and “the Hundred Years’ War.”

The plural followed by an apostrophe is also used in phrases like “ten dollars’ worth” or “five years’ experience” or “two days’ time.”

Apostrophe constructions like these aren’t “possessive” in the sense of ownership; strictly speaking, they’re genitive.”

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[found on http://www.grammarphobia.com/blog/2010/08/sui-genitive.html]