Nav Date Calculations

I've been asked to create a cashflow report by supplier based purely on purchase orders. To do that, I've been asked to use the expected receipt date as the "invoice date" and calculate the "payment date" using the payment terms. Has anyone managed to use the payment terms date calculation in Navision to do their own calculations? For example CM+30D for 30 days from end of month or 7D for 7 days. I could probably write something but that seems likely to be glitchy.