Rob, I'm pretty sure if you order 1 year, more than once you get years added on. I believe this because my expiration date is more than a year out and I contributed a couple times last year. You can also set up a monthly recurring subscription.
I know Keith didn't do this at first because it was all manual so they just did everyone on a calendar-year basis. Then the vBulletin software added subscription support and they went to using that (which co-incidentally tells you that by far this isn't the only forum that has a subscription model.)