When you purchase you already have all the points in your account until the end of that contract. The only thing is that you are limited to how far out you can access them.
Your UY always starts at the beginning of the referenced month and the date of your stay determines what points you can use. Every UY can always do their booking at the exact same time - 11 months from check in for a home resort and 7 months from check in at a non-home resort and the only variance is what UY they are able to use points from for those stays.
So, for example with a June UY and a check in on June 4, 2017 you would be able to use banked points from your 2016 UY (that were otherwise able to be used for stays from 6/1/16-5/30/17 but when banked are able to be used for stays from 6/1/17-5/31/18), current 2017 points (for 6/1/17-5/31/18 unless banked or borrowed) and you could borrow 2018 points (normally current for stays from 6/1/18-5/31/19 but if borrowed they are moved into your 2017 UY for stays from 6/1/17-5/31/18). You can call or go online to book that stay at a home resort on July 4, 2016 or for a non-home resort on Nov 4, 2016.
As another example - with a Juny UY and check in on May 4, 2017 you would be able to use banked 2015 points, current 2016 points or borrow 2017 points. For this stay you are able to book it beginning June 4, 2016 for a home resort or Oct 4, 2016 for a non-home resort.
This thread on understand UY might be helpful to read thru:
http://www.disboards.com/threads/understanding-use-year-updated-april-27-2016.1942668/