Under an obligation to pay interest, no rate being specified, interest is payable from date of incurrence of debt, unless the parties have otherwise agreed, at a maximum rate of the Category C rate of interest as established in § 54-3-16 , and in the like proportion for a longer or shorter term. In the computation of interest for less than a year, three hundred sixty days are deemed to constitute a year. Source: CivC 1877, § 1097; CL 1887, § 3720; RCivC 1903, § 1416; RC 1919, § 1039; SL 1933, ch 123 , § 1; SDC 1939, § 38.0108; SL 1980, ch 336 , § 1; SL 1982, ch 341 , § 2; SL 1984, ch 319 , § 34.