15 U.S.C. § 1666b: Timing of payments

A creditor may not treat a payment on a credit card account under an open end consumer credit plan as late for any purpose, unless the creditor has adopted reasonable procedures designed to ensure that each periodic statement including the information required by section 1637(b) of this title is mailed or delivered to the consumer not later than 21 days before the payment due date. If an open end consumer credit plan provides a time period within which an obligor may repay any portion of the credit extended without incurring an additional finance charge, such additional finance charge may not be imposed with respect to such portion of the credit extended for the billing cycle of which such period is a part, unless a statement which includes the amount upon which the finance charge for the period is based was mailed or delivered to the consumer not later than 21 days before the date specified in the statement by which payment must be made in order to avoid imposition of that finance charge.