So, the writer for Yahoo! Shine knew that there was something special about this expression (hmmm, maybe it requires some punctuation?), but wasn’t sure what. So, she came up with this:
Nice try. But wrong. Really wrong. The correct expression is “20 years’ worth.” It’s what the Associated Press Stylebook calls a “quasi possessive” — a possessive-like form. The Chicago Manual of Style calls it the “possessive with genitive.”According to Chicago, the genitive implies “of”: an hour’s delay (a delay of an hour). Whatever.
Here’s an easy way to figure out if you have a quasi possessive: Substitute “one” for the number. If you find that the possessive form sounds right, then you’ve got a quasi possessive. In this case, you’d try “one year’s worth” and because you have an ear for language, you know that the possessive is correct. Which brings me to the most important aspect of this method: You’ve got to have “an ear” for correct English. If you don’t, forget I told you this.