The AR Aging report breaks down the amount outstanding in issued Invoices for each Customer. 

The report is organized into 30 day buckets and breaks out the amounts owed by each customer in those buckets. The report will only pull from Invoices with the status of Issued or Partial Payment. The Created, Voided, and Paid status Invoices are not considered outstanding and therefore not taken into account for this report. The amounts represented in each bucket are calculated based off of the Invoice Date (e.g. Invoice Date of 5/15/2020 would be represented in the 31-60 bucket on 6/20/2020). Here is an example:

This means that the calculation for days from invoice date starts at the system date (e.g. system date set to 1/1/2020, invoice date of 12/20/2020 would be bucketed in the 0-30 range, even if today's date is 5/15/2020). This allows you to see the AR Aging of your customers from a specific date in time, making it easier to review historical data.