My last name begins with K; my plate's renewal date is June 21 of each year. If I buy a car now, my plate isn't just for 2015...it is good from now until June 21, 2016. The cost is pro-rated so I am paying for a partial year but also given the option to pay that partial year plus the next full year, making the plate good until June 21, 2017.
The plates are good from renewal date to renewal date, not for a full calendar year, January 1 to December 31. The renewal date is set by BMV based on your last name.
Edited to add that the BMV will also let you renew your driver's license up to a year in advance, so if you're there for something else but want to go ahead and get that renewal out of the way, you can.