Once activated, they are only good for one full year.
If you buy them now, but DON'T activate them, they will stay fresh and ready to activate at
any time (in 2017, if you like) and then be good for one full year following that eventual activation date.
What you will be buying is an Annual Pass VOUCHER.
When you are ready to use your AP, the voucher can be activated (converted to a "live AP") at any ticket booth or Guest Relations at WDW.