I would like to process payroll with a backdated pay date (for example, in Sep 2023), but QuickBooks Online Payroll is not allowing me to run payroll for previous dates.

Hi Pakking, it's great to have you here in the Community!

You can create a paycheck with a past check date as long as it's not a direct deposit and it's not after the tax period due date for paying taxes or filing forms for that period. 

If you receive a message to choose a more recent pay date while creating a paycheck, you can contact us for assistance.

For more detailed information, visit this article: Create a late (backdated) paycheck in QuickBooks Payroll.

In case you want to enter your employee's year-to-date paycheck info and company totals, you can check out this article for guidance: Add pay history to QuickBooks Online Payroll.

Let me know if you need further assistance processing payroll with a backdated pay date. I'm always available to help.