SDCL § 54-3-4: Maximum rate of interest where no rate specified--Commencement where date not specified.

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.