Customizing the A/R Aging Report to use Invoice Date

Hello,

I'd like to customize the vanilla A/R Aging report with 7-Buckets to use Invoice_date instead of due_date.   I've looked through the Oracle Report but cannot seem to locate where the computation of aging bucket amounts is performed.  Can someone please point me in the right direction as to where I might find the calculation of each bucket amount so I can make a custom version of this aging report and change the bucket calculations to use invoice_date instead of due_date?

The reason why we want to do this is we use the AR Aging report as a projection of future case in flows based on invoice date.   In the first bucket, I want to list the all invoices that are from 0 to 30 days old (not past due).