Thanks for the question.
ClinicOffice actually already has this feature built right in. Providing you raise the invoice from the appointment (right-click on the appointment record/from within the appointment itself) then ClinicOffice will ask if you want to back date the invoice.
Selecting ‘Yes’ uses the date of the appointment for the invoice as well as the due date, rather than todays date.
Please let us know if you have any further questions.