My apologies, I misread your original post and thought you were referring to back-dating invoices rather than preparing them in advance.
Unfortunately there is no way to automate this. You can manually set the dates each time, but the system assumes an invoice is created for when the patient owes you money, and therefore the appointment must have been completed, or payment must be required from that point.
You do have the option to adjust due dates to a fixed number of days in the future, but again, you cannot lock due dates to the date of the appointment automatically.
To automate the process would require a custom script, ergo a charge depending on the complexity, however, the option to do that would not always be desirable anyway, as appointment dates can change, and therefore this would introduce the possibility of unwanted accounting discrepancies.
As mentioned though, you can still override this manually and set the dates yourself.