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