When are you looking to travel? That’s the only thing that really matters.
If your stay is in January 2020, you’re in your 2019 UY and can only use the following points for that reservation:
Banked 2018 points
Current 2019 points
Borrowed 2020 points
One Time Use points at 7-months out
All the above points could be used between Sept 1, 2019 and Aug 31, 2020.