As long as the month of the invoice date matches the month of the posting date, it's fine. So if they're entering a November dated invoice as of December 1, they should just change the invoice date to 12/1. I know, I know - I've heard all the arguments but I've found the world doesn't come to an end if that happens. They can override the due date if they want but my argument is if they got it late, they should take full advantage of terms anyway.