Report Help - Starting/Ending Balance

I have a query calculates a running AR balance for multiple
physicians. Here is what I need to accomplish on a report.

Each physician has a record in the provider table that reflects their
initial Beginning AR balance and the date of the AR Balance. For
example, one physician has a Beginning AR date of 07/01/09. This
information is included in the query.

1.) How can I get the balance to display only for 07/01/09-07/31/09?
Any date/amount prior to the Beginning Date should be empty.
2.) How do I then take the Ending AR Balance on 07/31/09 and use that
as the Beginning Balance as of 08/01/09, etc. without writing it to a
table (if possible)