If you go to an existing Invoice and click Options -> Copy Invoice to create a new invoice as a copy of the existing invoice a dialog is shown for you to select the Client, Invoice Date, PDF password and Invoice Group.
If you alter the Invoice Date and click Submit the new invoice is created but the Invoice Date and the Due Date for the invoice is set to be the values that were in the original invoice that you copied from.
This is a bug I’m reporting, but I cant find any section specific for submitting bugs.
Edit: I am using v1.5.9