Possessives and Attributives

Q. Which is correct? (a) He has 15 years’ experience designing software, or (b) He has 15 years experience designing software. I’ve seen it written both ways. I believe “years” needs an apostrophe. If he has 15 years of experience, that would translate to “15 years’ experience.” Right? Please help.

A. You are absolutely right. Analogous to possessives, and formed like them, are certain expressions based on the old genitive case. As your question implies, the genitive here implies of. For some examples, see CMOS 7.25.