The A/R or A/P Aging Report does not display correct amounts (or the amount is different than the amounts displayed in Unpaid Bills or the Accounts Payable account on Balance Sheet or in register).
The as of report date setting in the Advanced Options of a report enables you run a report to show customer balances as of a selected date. Payments received after the ending date of the report are not reflected in the open balances shown on the report. Without this setting, payments after the report date would be reflected in the customer's balance.
For example: A customer has an invoice dated March 15th with a payment in full on April 10th, if you run the aging report for March 31st without selecting as of report date you would see a balance of 0 (the April 10th payment would be reflected in the report). If you change the setting to As of report Date the balance should be $500 (the actual balance on March 31st.)
There are other reasons this issue can be encountered, including damage in your data file.