As far as I know, there is no time limit. As you probably know, what you are actually purchasing at the DS is a "voucher" that needs to be exchanged at one of the parks or DD for the actual pass. The day you exchange the pass is the start of your 365 days. Again, I have never heard or read of a time limit to exchange the voucher for the actual pass.
Since Disney has your $$, I don't think they care when your year starts, LOL!