Summary field Total of restricted by subset of records
I have a multiple location inventory data base that I created. I need to generate reports that show how much inventory was in each location on a specific date in the past. I'm using a Sub summary report which shows Location and then item totals in that location. The item totals are generated by a summary field that totals the running balance. The table that it is based off of keeps track of each items movements in and out in a separate field for items in and items out then I have an item balance that subtracts the in from the out and then a summary field that totals.
How do I restrict this total to only total up to a certain date?