If an employee leaves in the 2020/2021 tax year and receives their final pay in the 2021/2022 tax year, for example, the employee leaves in tax month 12 in 2020/2021 and receives their final pay in tax month 1 in 2021/2022. The P45 must show the date in tax month 12 that the employee last worked for you.
You need to update the employee's final pay period in 2021/2022 as normal then set the process date into 2020/2021 to process the employee as a leaver and print the P45. You can then set the process date back into 2021/2022 to submit the full payment submission (FPS). The employee's information, including the leave date, is included in the FPS.