California Code, Civil Code - CIV § 1806.3

(1) If the finance charge was determined on the precomputed basis, the amount required to repay the contract shall be the outstanding contract balance as of that date, provided, however, that the buyer shall be entitled to a refund credit in the amount of the unearned portion of the finance charge.  The amount of the unearned portion of the finance charge shall be at least as great a proportion of the finance charge, or if the contract has been extended, deferred, or refinanced, of the additional charge therefor, as the sum of the periodic monthly time balances payable more than 15 days after the date of prepayment bears to the sum of all the periodic monthly time balances under the schedule of installments in the contract or, if the contract has been extended, deferred or refinanced, as so extended, deferred or refinanced.  Where the amount of the refund credit is less than one dollar ($1), no refund credit need be made by the holder.  Any refund credit in the amount of one dollar ($1) or more may be made in cash or credited to the outstanding obligations of the buyer under the contract.

(2) If the finance charge or a portion thereof was determined on the simple-interest basis, the amount required to prepay the contract shall be the outstanding contract balance as of that date, including any earned finance charges which are unpaid as of that date, and provided further that in cases where the finance charge or a portion thereof is determined on the 360-day basis, the payments theretofore received will be assumed to have been received on their respective due dates regardless of the actual dates on which those payments were received.