“Simple-interest basis” means the determination of a finance charge, other than an administrative finance charge, by applying a constant rate to the unpaid balance as it changes from time to time, either:
(a) Calculated on the basis of a 365-day year and actual days elapsed (although the seller may, but need not, adjust its calculations to account for leap years); reference in this chapter to the “365-day basis” shall mean this method of determining the finance charge, or
(b) For contracts entered into prior to January 1, 1988, calculated on the basis of a 360-day year consisting of 12 months of 30 days each and on the assumption that all payments will be received by the seller on their respective due dates; reference in this chapter to the “360-day basis” shall mean this method of determining the finance charge.