Running Backdated Inventory Reports

We are requested each by auditors to provide inventory reports for prior time periods.  As far as I am aware PS provides no methodology for running backdated inventory reports.  We currently try to save our current crystal reports each month for every business unit which is time consuming.  We also have crystal reports which take the current inventory and attempt to back out all of the transactions that have happened since the time period for the report date.  This has its pitfalls since transactions sometimes have issues.  I wish we could have an inventory table that contains the month end