You’ve been issued vouchers for the annual passes.
The clock doesn’t start ticking until you process them at guest services outside the parks or in DTD.
If you exchange the vouchers for passes on 3/28, that’s when your passes are created and that will be your anniversary date next year, the day your passes expire will be 3/28/16.
So yes, if you process on 3/28, your pass becomes active on that day and you can go right into the park.
I hope this answers your question.