The sticker doesn't matter. When they go to pull you over they look up your registration on their computer and it tells them when yours expires. Like the exact date. The sticker is there to give them a reason to possible check if you're expired. Like technically if you expire on on Sept 3rd, and your sticker says sept and it's sept 20th then it's expired.

This was a cop doing a power trip, if you don't believe that you're just not paying attention.