The start date of the fees refers to the day to which we set up the fee so that all of the clients are aligned to the same date and that attend to the periodicity of the fee.
By default we usually put 28 since they are the maximum days they have every month (including February). 

In this photo we can see the people assigned to a monthly fee set up and with the start date 28.

The advantage of setting day 28 of the month before day 1, for example, is that if we register the client in that fee last day 1, we will continue to offer the fee for that client that month (in fact, he will offer it to us until the day 28 of the month)
Regardless of the start date of the fee, we can issue it and collect it whenever we want, we can also edit it to change the price if it is the case.

The program considers a debt in a fee whenever the expiration date has passed, before that date the client can pay it, but the debt icon will not appear on the grid or in red in the collection tab of his client file.

Note: in the centers with integration of lathes in which there is a rule that denies access to customers with debts, the fee start day is usually advanced to day 5 so that the fee expiration takes place earlier.