Max Allowed Days determines how far back an invoice can be dated. This setting defines the maximum number of days in the past that users can backdate an invoice. For example, setting the value to 2,000 days allows invoices to be backdated by more than five years.

Another setting to consider is Prompt for Invoice Date When Posting. When enabled, users are prompted to enter an invoice date each time an invoice is posted. While this option can be useful when backdating invoices, it is generally recommended to keep it disabled during normal operations and enable it only when a backdated invoice needs to be posted.