Teamleader allows you to invoice in the past; but only until the date of your last invoice. The underlying reason for that is the automatic numbering system of our invoices.
Example: You booked your last invoice on September 25 and today is October 3. In this case, you can book invoices dating back as far as September 25.
Interesting piece of information is that you can still book invoices in the former fiscal year even though a new one has already begun. Yet, you need to keep the information just given above in mind.
Example: Your fiscal year is until December 31 and the last invoice was also booked on December 31. You can still book invoices for December 31 even though you booked one already in your new fiscal year, e.g. on January 2.