(a) Limitations on imposing finance charges as a result of the loss of a grace period -
(1) General rule. Except as provided in paragraph (b) of this section, a card issuer must not impose finance charges as a result of the loss of a grace period on a credit card account under an open-end (not home-secured) consumer credit plan if those finance charges are based on:
(i) Balances for days in billing cycles that precede the most recent billing cycle; or
(ii) Any portion of a balance subject to a grace period that was repaid prior to the expiration of the grace period.