I also have a Dec. use year.
I always book my trips 11 months ahead, use year impacts which points I can use for a particular visit, not when I can book a trip.
Any trip I take from 12/1/21 - 11/30/22 falls in my 2021 use year so I can use current 2021 points, banked 2020 points, and borrowed 2022 points (up to 50% at the moment) to make the reservation.
For example, last January I booked my December 2021 trip. I had some 2020 points I’d banked so I used those up plus I used 2021 points. I could have borrowed some 2022 points if I’d needed to, be didn’t have to this time.
The banking deadline for December is 7/31, btw.
If you have no 2021 points in the contract and want to travel from 1/1/22-11/30/22, then yes, you’d be borrowing from your 2022 points - w/ the current cap on borrowing you could borrow up to half your 2022 points.