In Florida, and I would hope since it makes sense, a lot of other states, your registration expires on your birthday. Always.
Say you buy a car in January.
Your birthday is in February.
You must pay the $35 or whatever to get your registration sticker.
When your birthday rolls around, you must purchase another registration as the one you got in january was only good until your next birthday.
There is really no excuse, in FL at least, for not knowing when your registration is up, if you dont know your own b-day you probably shouldnt be driving anyway.