ARSD § 64:44:01:05 — Determination of credit period

The first
day of a credit period is the day following the date of invoice, and the 30-day
period expires at midnight the thirtieth day following the date of invoice,
after which payment is delinquent. However, no remittance made by mail is
considered delinquent if it is affirmatively established that the envelope in
which the remittance was transmitted bears the postmark of the thirtieth day
following the date of invoice.