When late payment interest amount is added to a client under Clients -View Client/Edit tab, the system will calculate interest when an invoiceis downloaded (i.e., PDF icon is clicked ) meaning on that date andinterest is calculated on all unpaid balance.  The system will startfrom the oldest unpaid amount and calculate interest, then the systemwill iterate through all other unpaid invoices and calculate interest onunpaid balances.  Finally it will total it up and show the interestcharges on the downloaded invoice.