Tax question: declaring principal residence

This probably sounds like a dumb question, sorry in advance.

If I sold my principal residence in 2021 and bought a new principal residence in 2021 (which is typical), which one is to be designated as the "principal residence" for the 2021 tax year? I have to fill out the disposition date for property I sold and declare it was my residence in 2021.

So in a future year if I sell my new house and have to declare the years in which it was the principal residence, would it also be "2021" or does its principal residence status only start in "2022" even though it was purchased in 2021?