Aged Work in Process Report for a Previous Month

No - the Aged WIP Report is always a picture of WIP as of the day the report is run.  Although you can specify a transaction cutoff date, it is used only to exclude Work In Process dated after a certain date. When run, the report includes only those transactions dated on or before the specified “through” date that are unbilled at the time you run the report.  The report cannot go back and determine what was in WIP on 12/31.