From the Directory, page 336:
As you make your reservations, points will be deducted from the Use Year that your vacation begins.
So, as long as you are checking in on or before the expiration date, the entire reservation will be covered. Note, though, that if you are booking a "split" reservation, and the second (or subsequent) segments start after the end of your UY, those segments will consume points from the following UY.