I've done this before and yes, Belle is right. The AP is active from the date of first use, not the date of purchase. We got AP vouchers for Christmas from DBF's parents in Dec 08, we activated mine & DBF's early Jan 09, we activated the girls Feb 09. The passes were good for 1 year from the date of first use.
We didn't mind, MM wasn't a big deal for us so we were OK with not having that. Besides we did know that by staying on site we could get MM.