The payment due date for the American Express Credit Card payment varies between 18 days to  23 days from the billing date, depending on the nature of payment of the earlier statement dues. If the transaction date is 2nd May and the billing date is1st June, then the payment due date will be 22nd June. So the interest-free days will be from 2nd May to 22nd June i.e., 51 days if only the earlier credit card bill is paid in full instead of the minimum amount due.