(a)(1) Unless otherwise provided in this title, if any amount of tax imposed by this title (whether required on a return or to be paid by stamp or by some method) is not paid on or before the last date prescribed for payment, interest on the unpaid amount, at the underpayment rate set forth in subsection (d) of this section, shall be paid for the period from the last date prescribed for payment to the date paid.
(2)(A) The last date prescribed for payment shall be determined without regard to any extension of time for filing a return required under this title.
(B) In the case of taxes payable by stamp and in all other cases in which the last date for payment is not otherwise prescribed, the last date for payment shall be the date that the liability for the tax arises.
(b)(1) Except as provided under paragraphs (1) and (2) of this subsection, interest shall be imposed at the underpayment rate set forth in subsection (d) of this section on an assessable penalty or addition to the tax only:
(A) If the assessable penalty or addition to the tax is not paid within 21 calendar days after the date of notice and demand, and
(B) For the period from the date of the notice and demand to the date of payment.
(2) Interest shall be imposed at the underpayment rate set forth in subsection (d) of this section on an addition to tax imposed under §§ 47-4201, 47-4202, or 47-4203 [see § 47-4213] for the period which:
(A) Begins on the date of the return of the tax with respect to which the addition to tax imposed is required to be filed (including extensions); and
(B) Ends on the date of payment of the addition to tax.
(3) Interest shall not be imposed on an underpayment of estimated tax required to be paid under § 47-4215.
(c) Interest imposed under this section on an unpaid tax, assessable penalty, or addition to tax shall be paid upon notice and demand, and shall be assessed, collected, and paid in the same manner as a tax.
(d) The underpayment rate shall be as follows:
(1) 13% per year, simple interest, from January 1, 2001 to December 31, 2002;
(2) 10% per year, compounded daily, beginning January 1, 2003.
Section 410(b) of D.C. Law 13-305, as amended by section 36(b) of D.C. Law 14-213, provided: “Except as otherwise provided therein, sections 403 and 404 shall apply to taxes other than the real property tax imposed under Chapter 8 of Title 47, for all tax years or taxable periods beginning after December 31, 2000.”