P21 ages invoices by invoice date and places each open balance into a time bucket based on how many days have passed. The buckets are: Current (0–29 days), 30–59, 60–89, 90–119, 120–149, and 150+ days.

Current Due is the amount not yet past due — invoices where fewer than 30 days have elapsed since the invoice date. The Total column sums all buckets, giving you the complete open balance for each customer regardless of age.

Buckets can be reconfigured in P21 under AR Options to use due date instead of invoice date, which shifts balances based on the customer's payment terms rather than when the invoice was issued.

DSO measures the average number of days a customer takes to pay after being invoiced. It is calculated using the P21 built-in function p21_fn_days_sales_outstanding, which compares each customer's open AR balance against their average daily sales over the configured AR window (default: 30 days).

Formula: DSO = Open AR ÷ Average Daily Sales

A customer with a DSO higher than their stated payment terms is a collection risk. For example, a customer on Net 30 terms with a DSO of 55 is consistently paying 25 days late. Track DSO trends over time — a rising DSO is an early warning sign before balances move into older aging buckets.