You can arrive at the Apply Payment page through a couple of ways:
Accounts Receivable > Payments > Deposits > Batch # > (Deposit) # > Apply button
CRM > Customers > Payments tab > $ Apply button in a grid row.
The Applied Date field defaults to the current date, and you can change the date to any valid date.
For GL Entry integration, the applied date is used as the entry date for all GL entries related to the applied payment.
For credits, the applied date follows the same behavior as payments; the field defaults to the current date and users can change the date to any valid date.
There is a confirmation message if you enter an applied date outside the open accounting period.