why does the payroll pay schedule nest payday not accept a date?

Once an employee has been paid based on a specific pay schedule, the Next Payday field is locked. The system does this to protect your past payroll and tax history from incidental errors.

To utilize your desired next pay date, you might consider creating a new schedule and assigning your existing employee to it. For a complete process, check out this article: Set up and manage pay schedules in QuickBooks Online Payroll.

Let me know if you have further questions.