License sticker green tag?

A car a bought from my dad awhile ago had new stickers on I think was sometime in late winter or early spring can't really remember which.
Tonight when I was driving home after work I was pulled over. The officer asked "Did you know your tag was expired?"
I said No which was the truth.
How does one know when tags expire? I have never been shown how to know when tags expire I thought the L 2017 stood for December 2017 this entire time. Is that not how it works?
Asked Sep 13, 2017

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