What is the timing for claiming the PTET Credit - based on the payment year, or the election year?

In other words, when the election is timely made (by 3/15/2022) for the 2021 PTE tax return, when is the credit available on the 1040

(a) for someone who paid the PTET in 2021; and

(b) for someone who paid the PTET in 2022 (before 3/15) ?

Still in the PTET maze....