If you extend out your payment date. From say the 10th to the 20th. You will be changing your due payment date. Your interest that posts to the mortgage will still post on the same day as previously. So technically your balance is higher until you make your payment. This higher balance is now the balance that is being used to calculate the interest. So Interest will be higher. As their systems now calculate daily. Once your payment is made the new balance is used to calculate the interest till the next month when it occurs again. Repeat and rinse to the end.