It takes the total number of days to pay all invoices in the system and divides by the total number of invoices. For example, a customer had five invoices and they paid the invoices in 28, 22, 15, 35, and 20 days. Sage 50 will add these five numbers and divide by 5, giving an average days to pay of 24 days.
Average days to pay is recalculated for a customer whenever an invoice for that customer is closed.
When the invoice has been paid in full, either with Receipts or Credit Memos, its status is changed to closed.