I am having some trouble calculating interest owed to me from an operator in New Mexico using the US Interest Rule. NM statute states that operators have 45 days at the end of the month of production to send payments to interest owners and failure to do so will have a penalty of 18% per annum. Now this has been a recurring problem and after numerous conversations with the operator to pay on time, they failed to do so. I'm using an excel spreadsheet to calculate interest using the U.S. interest rule but I'm getting stuck with my formula because I seem to be accruing interest which is not allowed under the U.S. interest rule. Can anyone shed light on a possible formula that I can use?