26 USC 6601 - Interest on underpayment, nonpayment, or extensions of time for payment, of tax

(a) General rule 
If any amount of tax imposed by this title (whether required to be shown on a return, or to be paid by stamp or by some other method) is not paid on or before the last date prescribed for payment, interest on such amount at the underpayment rate established under section 6621 shall be paid for the period from such last date to the date paid.
(b) Last date prescribed for payment 
For purposes of this section, the last date prescribed for payment of the tax shall be determined under chapter 62 with the application of the following rules:
(1) Extensions of time disregarded 
The last date prescribed for payment shall be determined without regard to any extension of time for payment or any installment agreement entered into under section 6159.
(2) Installment payments 
In the case of an election under section 6156 (a)1 to pay the tax in installments

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(A) The date prescribed for payment of each installment of the tax shown on the return shall be determined under section 6156 (b),1 and
(B) The last date prescribed for payment of the first installment shall be deemed the last date prescribed for payment of any portion of the tax not shown on the return.
(3) Jeopardy 
The last date prescribed for payment shall be determined without regard to any notice and demand for payment issued, by reason of jeopardy (as provided in chapter 70), prior to the last date otherwise prescribed for such payment.
(4) Accumulated earnings tax 
In the case of the tax imposed by section 531 for any taxable year, the last date prescribed for payment shall be deemed to be the due date (without regard to extensions) for the return of tax imposed by subtitle A for such taxable year.
(5) Last date for payment not otherwise prescribed 
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 deemed to be the date the liability for tax arises (and in no event shall be later than the date notice and demand for the tax is made by the Secretary).
(c) Suspension of interest in certain income, estate, gift, and certain excise tax cases 
In the case of a deficiency as defined in section 6211 (relating to income, estate, gift, and certain excise taxes), if a waiver of restrictions under section 6213 (d) on the assessment of such deficiency has been filed, and if notice and demand by the Secretary for payment of such deficiency is not made within 30 days after the filing of such waiver, interest shall not be imposed on such deficiency for the period beginning immediately after such 30th day and ending with the date of notice and demand and interest shall not be imposed during such period on any interest with respect to such deficiency for any prior period. In the case of a settlement under section 6224 (c) which results in the conversion of partnership items to nonpartnership items pursuant to section 6231 (b)(1)(C), the preceding sentence shall apply to a computational adjustment resulting from such settlement in the same manner as if such adjustment were a deficiency and such settlement were a waiver referred to in the preceding sentence.
(d) Income tax reduced by carryback or adjustment for certain unused deductions 

(1) Net operating loss or capital loss carryback 
If the amount of any tax imposed by subtitle A is reduced by reason of a carryback of a net operating loss or net capital loss, such reduction in tax shall not affect the computation of interest under this section for the period ending with the filing date for the taxable year in which the net operating loss or net capital loss arises.
(2) Foreign tax credit carrybacks 
If any credit allowed for any taxable year is increased by reason of a carryback of tax paid or accrued to foreign countries or possessions of the United States, such increase shall not affect the computation of interest under this section for the period ending with the filing date for the taxable year in which such taxes were in fact paid or accrued, or, with respect to any portion of such credit carryback from a taxable year attributable to a net operating loss carryback or a capital loss carryback from a subsequent taxable year, such increase shall not affect the computation of interest under this section for the period ending with the filing date for such subsequent taxable year.
(3) Certain credit carrybacks 


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(A) In general 
If any credit allowed for any taxable year is increased by reason of a credit carryback, such increase shall not affect the computation of interest under this section for the period ending with the filing date for the taxable year in which the credit carryback arises, or, with respect to any portion of a credit carryback from a taxable year attributable to a net operating loss carryback, capital loss carryback, or other credit carryback from a subsequent taxable year, such increase shall not affect the computation of interest under this section for the period ending with the filing date for such subsequent taxable year.
(B) Credit carryback defined 
For purposes of this paragraph, the term credit carryback has the meaning given such term by section 6511 (d)(4)(C).
(4) Filing date 
For purposes of this subsection, the term filing date has the meaning given to such term by section 6611 (f)(4)(A).
(e) Applicable rules 
Except as otherwise provided in this title
(1) Interest treated as tax 
Interest prescribed under this section on any tax shall be paid upon notice and demand, and shall be assessed, collected, and paid in the same manner as taxes. Any reference to this title (except subchapter B of chapter 63, relating to deficiency procedures) to any tax imposed by this title shall be deemed also to refer to interest imposed by this section on such tax.
(2) Interest on penalties, additional amounts, or additions to the tax 

(A) In general 
Interest shall be imposed under subsection (a) in respect of any assessable penalty, additional amount, or addition to the tax (other than an addition to tax imposed under section 6651 (a)(1) or 6653 or under part II of subchapter A of chapter 68) only if such assessable penalty, additional amount, or addition to the tax is not paid within 21 calendar days from the date of notice and demand therefor (10 business days if the amount for which such notice and demand is made equals or exceeds $100,000), 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.
(B) Interest on certain additions to tax 
Interest shall be imposed under this section with respect to any addition to tax imposed by section 6651 (a)(1) or 6653 or under part II of subchapter A of chapter 68 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.
(3) Payments made within specified period after notice and demand 
If notice and demand is made for payment of any amount and if such amount is paid within 21 calendar days (10 business days if the amount for which such notice and demand is made equals or exceeds $100,000) 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.
(f) Satisfaction by credits 
If any portion of a tax is satisfied by credit of an overpayment, then no interest shall be imposed under this section 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. The preceding sentence shall not apply to the extent that section 6621 (d) applies.
(g) Limitation on assessment and collection 
Interest prescribed under this section on any tax may be assessed and collected at any time during the period within which the tax to which such interest relates may be collected.
(h) Exception as to estimated tax 
This section shall not apply to any failure to pay any estimated tax required to be paid by section 6654 or 6655.
(i) Exception as to Federal unemployment tax 
This section shall not apply to any failure to make a payment of tax imposed by section 3301 for a calendar quarter or other period within a taxable year required under authority of section 6157.
(j) 2-percent rate on certain portion of estate tax extended under section 6166 

(1) In general 
If the time for payment of an amount of tax imposed by chapter 11 is extended as provided in section 6166, then in lieu of the annual rate provided by subsection (a)
(A) interest on the 2-percent portion of such amount shall be paid at the rate of 2 percent, and
(B) interest on so much of such amount as exceeds the 2-percent portion shall be paid at a rate equal to 45 percent of the annual rate provided by subsection (a).

For purposes of this subsection, the amount of any deficiency which is prorated to installments payable under section 6166 shall be treated as an amount of tax payable in installments under such section.

(2) 2-percent portion 
For purposes of this subsection, the term 2-percent portion means the lesser of
(A) 
(i) the amount of the tentative tax which would be determined under the rate schedule set forth in section 2001 (c) if the amount with respect to which such tentative tax is to be computed were the sum of $1,000,000 and the applicable exclusion amount in effect under section 2010 (c), reduced by
(ii) the applicable credit amount in effect under section 2010 (c), or
(B) the amount of the tax imposed by chapter 11 which is extended as provided in section 6166.
(3) Inflation adjustment 
In the case of estates of decedents dying in a calendar year after 1998, the $1,000,000 amount contained in paragraph (2)(A) shall be increased by an amount equal to
(A) $1,000,000, multiplied by
(B) the cost-of-living adjustment determined under section 1 (f)(3) for such calendar year by substituting calendar year 1997 for calendar year 1992 in subparagraph (B) thereof.

If any amount as adjusted under the preceding sentence is not a multiple of $10,000, such amount shall be rounded to the next lowest multiple of $10,000.

(4) Treatment of payments 
If the amount of tax imposed by chapter 11 which is extended as provided in section 6166 exceeds the 2-percent portion, any payment of a portion of such amount shall, for purposes of computing interest for periods after such payment, be treated as reducing the 2-percent portion by an amount which bears the same ratio to the amount of such payment as the amount of the 2-percent portion (determined without regard to this paragraph) bears to the amount of the tax which is extended as provided in section 6166.
(k) No interest on certain adjustments 
For provisions prohibiting interest on certain adjustments in tax, see section 6205 (a).
[1] See References in Text note below.