27 CFR § 70.90 - Interest on underpayments.
(a) General rule. Interest at the underpayment rate referred to in § 70.93 of this part shall be paid on any unpaid amount of tax from the last date prescribed for payment of the tax (determined without regard to any extension of time for payment) to the date on which payment is received.
(b) Interest on penalties, additional amounts, or additions to the tax -
(1) General. Interest shall be imposed on any assessable penalty, additional amount, or addition to the tax (other than an addition to tax imposed under section 6651(a)(1) of the Internal Revenue Code) only if such assessable penalty, additional amount, or addition to the tax is not paid within 10 days from the date of notice and demand therefor, and in such case interest shall be imposed only for the period from the date of the notice and demand to the date of payment.
(2) Interest on certain additions to tax. Interest shall be imposed under this section on any addition to tax imposed by section 6651(a)(1) of the Internal Revenue Code for the period which (i) begins on the date on which the return of the tax with respect to which such addition to tax is imposed is required to be filed (including any extensions), and (ii) ends on the date of payment of such addition to tax.
(c) Payments made within 10 days after notice and demand. If notice and demand is made for payment of any amount, and if such amount is paid within 10 days after the date of such notice and demand, interest under this section on the amount so paid shall not be imposed for the period after the date of such notice and demand.
(d) Satisfaction by credits. If any portion of a tax is satisfied by credit of an overpayment, then no interest shall be imposed under section 6601 of the Internal Revenue Code on the portion of the tax so satisfied for any period during which, if the credit had not been made, interest would have been allowable with respect to such overpayment.
(e) Last date prescribed for payment.
(1) In determining the last date prescribed for payment, any extension of time granted for payment of tax shall be disregarded. The granting of an extension of time for the payment of tax does not relieve the taxpayer from liability for the payment of interest thereon during the period of the extension. Thus, except as provided in paragraph (d) of this section, interest at the underpayment rate referred to in § 70.93 of this part is payable on any unpaid portion of the tax for the period during which such portion remains unpaid by reason of an extension of time for the payment thereof.
(2) In the case of taxes payable by stamp and in all other cases where the last date for payment of the tax is not otherwise prescribed, such last date for the purpose of the interest computation shall be deemed to be the date on which the liability for the tax arose. However, such last date shall in no event be later than the date of issuance of a notice and demand for the tax.
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