The expiration date will be 1 year from the day you activate it.
If you order the pass online, you will receive a voucher. Before you can get into the parks, you need to exchange the voucher for your actual ticket. The countdown to expiration begins when you make that exchange.
Also - when you DO activate your pass - be VERY mindful of the expiration date on the back! It happened to me, and several other DISers - they printed the WRONG expiration date on it. Some have experienced an expiration date from when they bought the VOUCHER - in my instance, the expiration date listed was the exact same day as activation.
So just make sure you check that out before you leave the line!!!!