Subject to the other provisions of §§ 4315-4317 of this title, the service charge to be included in a consolidated time balance shall be determined by applying the service charge at the applicable rate specified in that article to either:
(1) The total of the unpaid balance of the subsequent contract and the unpaid balance of any previous contract included in the consolidated total determined by deducting from the then unpaid time balance thereof any then unearned service charge in an amount not less than the refund credit for anticipation in §§ 4320-4323 of this title (computed, however, without the allowance of any minimum earned service charge), for the period from the date thereof to and including the date when the final installment of such consolidated total is payable; or
(2) The principal balance of the subsequent contract for the period from the date thereof to and including the date when the final installment of such consolidated total is payable and, if the due date of the final installment of such consolidated total is later than the due date of the final installment of any previous contract included in the consolidated total, on the time balance then unpaid on such previous contract from the date when the final installment thereof was payable to the date when the final installment of such consolidated total is payable.