invoice dates

Hi, I tend to write invoices and update my database with payments received once every two weeks, rather than on the same day as the treatment. The problem I am currently having is that the invoice shows only the day the invoice is created, and NOT the date of the session for which I am creating the invoice. Thus, if I catch up and write, lets say invoices and register payment for the last 2 weeks today, I have 3 invoices with todays date for patient X, and it is not really clear for which sessions.

How do I get around this problem?

PS. I am brand new to this