I am a bit surprised that you are using the system date for your pro rata calculations. I have been working on a number of billing systems, they all made a clear and important difference between the (theoretical) bill date and the actual date of the bill run, which might often be one or two days after the bill date. If a customer is supposed to be billed on the 8 of each month, the billing services are supposed to go from the 8th of one month to the 7th of next month, irrespective of whether you actually run the billing on the 8th, the 9th or perhaps even the 10th of the current month.