It depends on where you purchase it.
If you purchase it while in WDW at a Guest Relations location, it will expire one year from the date you purchased it.
If you purchase it from Member Services, on the other hand, they send you a voucher, not the actual annual pass. Your purchase price is locked in at what you paid, but you'll still need to visit a Guest Relations location to exchange the voucher for your annual pass. The pass will expire one year from the date you do the exchange.
As previously posted, it will expire on the same date the next year (as opposed to expiring 365 days later).