Because BookingCenter works on a System Date concept, if you desire to manually add bookings to dates in the past, it must be done by altering the System Date to re-create the activity you wish to record in the PMS.
Doing this System Date over-ride can only be done with BookingCenter's Support Dept help. While the System Date is in the 'past' it is imperative that staff not make bookings, charge folios, or make payments for current bookings. Otherwise, the mix up of 'past' and 'present' activity will become difficult to reconcile.
For example, if today is July 5 and you wish to add bookings (and folio charges and payments) from May 1-10, then you would choose a time that staff activity is slow, notify staff that they cannot use the system for an hour or so for activity relating to today (July 5) bookings, and then contact the Support Dept at BookingCenter to set the System Date to May 1. With the System Date now set to May 1, you would enter any bookings, charges, and payments and perform Night Audit from May 1- 10 (ie, 9 times) until the work desired to do was accomplished. Then, you would contact the Support Dept at BookingCenter to set the System Date to 'today' again, July 5.
By doing this, the transaction logs will show that all bookings, charges, and payments were done on July 5 (ie, the real user activity will show what actually is happening on July 5), but the work will be reported for May 1-10, as desired.
Please plan such work ahead of time, so that you have enough time to do what is desired, as while the System Date has been over-ridden by the Support Dept at BookingCenter, if users enter the system to work, their work will apply to the 'wrong' dates. So, be careful.
Note that any online bookings will not be affected by the over-ride of the System Date, so don't worry about turning off your Booking Engine or GDS/OTA profiles.