South Dakota Codified Laws
Chapter 11 - Credit Cards And Revolving Charge Accounts
Section 54-11-6.3 - Maximum service charge.

54-11-6.3. Maximum service charge.
The charge shall be at a rate or amount agreed upon by the parties involved. If the billing cycle is not monthly, the maximum charge is that percentage which bears the same relation to the agreed upon rate as the number of days in the billing cycle bears to thirty. For the purposes of this section, a variation of not more than four days from month to month is approximately the same day of the billing cycle.

Source: SL 1979, ch 335, §5; SL 1980, ch 337, §1A; SL 1982, ch 346, §1; SL 1987, ch 360, §5.