The system calculates the due date individually for a single pre-bill booking or monthly for long running pre-bill bookings.

The following three fields are used in the calculation.

1. When you set your booking Is prebill = Yes (Booking > Main tab) the calculation triggers.
2. Verify the default billing date.  From the Agency setup menu, select Configuration. Check the Default billing date fields. 
3. Verify the vendor lead time.  From the Finance menu, select Vendor contracts. Check the Lead time to air (days) value and the Contract specific option is selected.
4. Verify the client payment terms.  From your client invoice, check the Payment terms value.

From these values, the system calculates the pre-bill Client billings due date.