S.C. Code Ann. § 12-54-25 — Interest due on late taxes; applicable interest rate

(A) If any tax is not paid when due, interest is due on the unpaid portion from the time the tax was due until paid in its entirety. For administrative convenience, the department may waive up to thirty days' interest.
(B) For purposes of this section, a tax is due on the last day provided for its payment, without regard for any extension of time for payment and without regard for or to any assessment under Section 12-60-910. Stamp taxes and any other tax for which no payment date is provided are due on the day the liability arises.
(C)(1) Any tax refunded or credited must include interest on the amount of the credit or refund from the latest of the date the tax was paid, the original due date of the return, or the last day prescribed for paying the tax if no return is required, to either the date the refund was sent or delivered to the taxpayer or the date the credit was made.
(2) This interest must be paid by drawing upon funds from the type of tax being refunded or credited. The funds withdrawn may be expended by the department in the payment of interest on refunds.
(3) Interest on an overpayment is not allowed pursuant to this subsection if it is refunded:
(a) within seventy-five days after the last day prescribed for filing the tax return, without regard to an extension of time for filing, or within seventy-five days after the last day prescribed for paying the tax if no return is required;
(b) within seventy-five days after the return is filed, in the case of a return filed after the last date;
(c) within seventy-five days after the taxpayer files a claim for a credit or refund for the overpayment of tax for the period between the filing of the claim to the payment of the refund; or
(d) within seventy-five days after the county has received notice from the Department of Revenue that the taxpayer is due a credit or refund for the overpayment of property taxes.
(D) Except as preempted or superseded by federal law or inter-governmental compact such as the International Fuel Tax Agreement, the rate of interest on underpayments and overpayments is established by the department in the same manner and at the same time as the underpayment rate provided in Internal Revenue Code Sections 6621(a)(2) and 6622.