(a) If the first installment due date is one month or less from the date of the loan, the authorized lender may retain for each elapsed day between the date of the loan and prepayment before the first installment due date, 1/30th of the installment account handling charge that could be retained if the first installment period were one month and the loan was prepaid in full on the first installment due date. All interest in excess of such amount must be refunded or credited to the borrower.(b) If the first installment due date is more than one month from the contract date but less than or equal to one month and 15 days from the contract date, the authorized lender may retain for each elapsed day between the date of the loan and prepayment before the first installment due date, 1/30th of the interest that could be retained if the first installment period were one month and the loan was prepaid in full on the first installment due date up to a maximum of 30 days. All interest in excess of such amount must be refunded or credited to the borrower.(c) To calculate the amount of the refund of unearned interest, an authorized lender must consider any installments that were deferred.