When you customize a report, use the aging settings to find out which bills are overdue and by how long. You can also calculate this information as of any date, not just today.
Since A/R, Trade and other receivables, Debtors, or A/R (Debtors) aging reports are more common, customers are discussed here, although the same information applies to vendors when you run other payable or creditor aging reports.
Aging "as of" what date?
To calculate each customer's open balance based on all payments received, set Aging Method to Current.
Or, to calculate open balances based on payments received as of the transaction date of the report, set Aging Method to Report Date.
Set the As of date to the date you want. (You can use the Transaction Date list to change the As of date, or you can set the As of date directly.)
For example, to find out which customers were late in making payments last year, set the Aging Method to Report Date and set the Transaction Date to Last Year.