Have you looked at the Timeslips area of the program?  It might be suitable to your needs though it is only available in Premium and Quantum.

If you are using Pro, you might want to look at using a Quote (or Order if you want it to show in the Pending report) for each client and just keep adding time to it as you move through the billing cycle.  Then when you are at the end of a cycle, convert to an invoice and start another quote/order for the next period.