(a) Section 21 applies to all taxpayers, including individuals and corporations. It provides a general rule applicable in any case where (1) any rate of tax imposed by chapter 1 of the Code upon the taxpayer is increased or decreased, or any such tax is repealed, and (2) the taxable year includes the effective date of the change, except where that date is the first day of the taxable year. For example, the normal tax on corporations under section 11(b) was decreased from 30 percent to 22 percent in the case of a taxable year beginning after December 31, 1963. Accordingly, the tax for a taxable year of a corporation beginning on January 1, 1964, would be computed under section 11(b) at the new rate without regard to section 21. However, for any taxable year beginning before January 1, 1964, and ending on or after that date, the tax would be computed under section 21. For additional circumstances under which section 21 is not applicable, see paragraph (k) of this section.

(b) In any case in which section 21 is applicable, a tentative tax shall be computed by applying to the taxable income for the entire taxable year the rate for the period within the taxable year before the effective date of change, and another tentative tax shall be computed by applying to the taxable income for the entire taxable year the rate for the period within the taxable year on or after such effective date. The tax imposed on the taxpayer is the sum of—

(1) An amount which bears the same ratio to the tentative tax computed at the rate applicable to the period within the taxable year before the effective date of the change that the number of days in such period bears to the number of days in the taxable year, and

(2) An amount which bears the same ratio to the tentative tax computed at the rate applicable to the period within the taxable year on and after the effective date of the change that the number of days in such period bears to the number of days in the taxable year.

(c) If the rate of tax is changed for taxable years “beginning after” or “ending after” a certain date, the following day is considered the effective date of the change for purposes of section 21. If the rate is changed for taxable years “beginning on or after” a certain date, that date is considered the effective date of the change for purposes of section 21. This rule may be illustrated by the following examples:

(d) If a tax is repealed, the repeal will be treated as a change of rate for purposes of section 21, and the rate for the period after the repeal (for purposes of computing the tentative tax with respect to that period) will be considered zero. For example, the Tax Reform Act of 1969 repealed section 1562, which imposed a 6 percent additional tax on controlled corporations electing multiple surtax exemptions, effective for taxable years beginning after December 31, 1974. For such controlled corporations having taxable years beginning in 1974 and ending in 1975, the rate for the period ending before January 1, 1975, would be 6 percent; the rate for the period beginning after December 31, 1974, would be zero. However, subject to the rules stated in this section, section 21 does not apply to the imposition of a new tax. For example, if a new tax is imposed for taxable years beginning on or after July 1, 1972, a computation under section 21 would not be required with respect to such new tax in the case of taxable years beginning before July 1, 1972, and ending on or after that date. If the effective date of the imposition of a new tax and the effective date of a change in rate of such tax fall in the same taxable year, section 21 is not applicable in computing the taxpayer's liability for such tax for such year unless the new tax is expressly imposed upon the taxpayer for a portion of his taxable year prior to the change in rate.

(e) If a husband and wife have different taxable years because of the death of either spouse, and if a joint return is filed with respect to the taxable year of each, then, for purposes of section 21, the joint return shall be treated as if the taxable years of both spouses ended on the date of the closing of the surviving spouse's taxable year. See section 6013 (c), relating to treatment of joint return after death of either spouse. Accordingly, if a change in the rate of tax is effective during the taxable year of the surviving spouse, the tentative taxes with respect to the joint return shall be computed on the basis of the number of days during which each rate of tax was in effect for the taxable year of the surviving spouse.

(f) Section 21 applies whether or not the taxpayer has a taxable year of less than 12 months. Moreover, section 21 applies whether or not the taxable income for a taxable year of less than 12 months is required to be placed on an annual basis under section 443. If the taxable income is required to be computed under section 443(b) then the tentative taxes under section 21 are computed as provided in paragraph (1) or (2) of section 443(b) and are reduced as provided in those paragraphs. The tentative taxes so computed and reduced are then apportioned as provided in section 21(a)(2) to determine the tax for such taxable year as computed under section 21.

(g) If a taxpayer has made the election under section 441(f) (relating to computation of taxable income on the basis of an annual accounting period varying from 52 to 53 weeks), the rules provided in section 441(f)(2) shall be applicable for purposes of determining whether section 21 applies to the taxable year of the taxpayer. Where a taxpayer has made the election under section 441(f) and where section 21 applies to the taxable year of the taxpayer the computation under section 21(a)(2) shall be made upon the basis of the actual number of days in the taxable year and in each period thereof.

(h)

(1) Section 21 is applicable only if the rate of tax imposed by chapter 1 changes. Sections in which rates of tax are specified or incorporated by reference include the following: 1, 2, 3, 11, 511, 531, 541, 821, 831, 871, 881, 1201, and 1348 (for taxable years beginning after December 31, 1970). Except as provided in subparagraph (3) of this paragraph, section 21 is not applicable with respect to changes in the law relating to deductions from gross income, exclusions from or inclusions in gross income, or other items taken into account in determining the amount or character of income subject to tax. Moreover, section 21 is not applicable with respect to changes in the law relating to credits against the tax or with respect to changes in the law relating to limitations on the amount of tax. Section 21 is applicable, however, to all those computations specified in the section providing the rate of tax which are implicit in determining the rate. For example, if one of the tax brackets in the tax tables under section 3 were to be changed, section 21 would be applicable to that change. Thus, if the bracket relating to “at least $4,200 but not less than $4,250” for heads of households should be changed to increase or decrease the last sum specified, with corresponding changes being made in subsequent brackets, section 21 would be applicable. The enactment of sections 1561 and 1562 is considered a change in section 11(d) which constitutes a change in rate for the period ending after December 31, 1963. The amendment of section 1561 and the repeal of section 1562 by the Tax Reform Act of 1969 is considered a change in section 11(d) which constitutes a change in rate for the period ending after December 31, 1974. The repeal of the 2 percent additional tax imposed under section 1503 on corporations filing consolidated returns constitutes a change in rate for the period ending after December 31, 1963. The addition to the Code of section 1348 (relating to 50 percent maximum rate on earned income) is a change in rate to which section 21(a) is applicable. The amendment of section 11(d) by the Tax Reduction Act of 1975 which increases to $50,000 the surtax exemption for a taxable year ending during 1975 constitutes a change in rate for such portion of the taxable year (if less than the entire taxable year) as follows December 31, 1974. Similarly, the return of the surtax exemption to $25,000 for a taxable year ending during 1976 constitutes a change in rate for such portion of the taxable year (if less than the entire taxable year) as follows December 31, 1975.

(2) Ordinarily, both the old and the new rates are applied to the same amount of taxable income. However, where the rate of tax is itself taken into account in determining taxable income (for example, the special deduction for Western Hemisphere trade corporations under section 922), the taxable income used in determining the tentative tax employing the rate before the effective date of change shall be determined by reference to that rate of tax, and the taxable income for the purpose of determining the tentative tax employing the rate for the period on and after the effective date of the change shall be determined by reference to the new tax rate.

(3) Section 21 is applicable with respect to changes in the law relating to the standard deduction for individuals provided in part IV of subchapter B and to the deduction for personal exemptions for individuals provided in part V of subchapter B.

(i) If the rate of tax changes more than once during the taxable year, section 21 is applicable to each change in rate. For example, if the rate of normal tax changed for taxable years beginning on or after March 1, 1954, and changed again for taxable years beginning on or after June 1, 1954, section 21 requires computation of 3 tentative taxes for any taxable year which began before March 1, 1954, and ended on or after June 1, 1954: One tentative tax at the rate in effect before the March 1 change; another tentative tax at the rate in effect from March 1 to May 31; and a third tentative tax at the rate in effect from June 1 to the end of the taxable year. The proportion of each such tentative tax taken into account in determining the tax imposed on the taxpayer is computed by reference to the portion of the taxable year before March 1, 1954, by reference to the portion of the taxable year from March 1, 1954, through May 31, 1954, and by reference to the portion of the taxable year from June 1, 1954, to the end of the taxable year, respectively.

(j)

(1) If a change in the rate of one tax imposed by chapter 1 of the Code does not affect the amount of other taxes imposed by chapter 1 of the Code the other taxes may be determined without regard to section 21 and section 21 will be applied only to the tax for which a change in rate is made. However, if the change of rate of one tax does affect the amount of other taxes imposed under chapter 1 of the Code, then the computation of the taxes under chapter 1 of the Code so affected shall be made by applying section 21. For example, if section 1201 applies to an individual taxpayer for a taxable year containing the effective date of a change in a rate of tax provided in section 1, then under section 21 the taxpayer must compute a tentative tax for each period for which a different rate of tax is effective under section 1. The tentative tax for each such period as computed under section 1201 will reflect the rate of tax provided by section 1 for such period.

(2) In certain cases chapter 1 of the Code provides that the particular tax to be imposed upon the taxpayer shall be one of several taxes, the basis of selection being the tax that is greater or lesser. See, for example, sections 821 and 1201. If in any such case the rate of any one of these taxes changes, then the tentative taxes computed as provided by section 21 for each period shall be computed employing the tax selected in accordance with the general rule of selection for such a case, at the rate of tax in effect for such period. Thus, if a change in the rate of the alternative tax under section 1201 is such that the alternative tax under section 1201 is applicable if the old rate is used and is not applicable if the new rate is used, one tentative tax will consist of the alternative tax under section 1201 and the other tentative tax will consist of the tax imposed by the other applicable sections of chapter 1 of the Code. The two tentative taxes so computed are then prorated in accordance with section 21(a)(2) and the sum of the proportionate amounts is the tax imposed for the taxable year under chapter 1 of the Code. See the examples in paragraph (n) of this section.

(k) Section 21 does not apply in the following situations:

(1) The provisions of section 21 do not apply to the imposition of the tax surcharge by section 51. The proration rules of section 51(a) apply in the case of a taxable year ending on or after the effective date of the surcharge and beginning before July 1, 1970.

(2) The provisions of section 21 do not apply to the imposition of the minimum tax for tax preferences by section 56. The proration rules of section 301(c) of the Tax Reform Act of 1969 (83 Stat. 586) apply in the case of a taxable year beginning in 1969 and ending in 1970.

(l) In computing the number of days each rate of tax is in effect during the taxable year for purposes of section 21(a)(2), the effective date of the change in rate shall be counted in the period for which the new rate is in effect.

(m) Any credits against tax, and any limitation in any credit against tax, shall be based upon the tax computed under section 21. For credits against tax, see part IV (section 31 and following), subchapter A, chapter 1 of the Code.

(n) The application of section 21 may be illustrated by the following examples: (See also the examples in §1.1561-2A(a)(3).)

(Secs. 1561(a) (83 Stat. 599; 26 U.S.C. 1561(a)) of the Internal Revenue Code)

[T.D. 6500, 25 FR 11402, Nov. 26, 1960; 25 FR 14021, Dec. 31, 1960, as amended by T.D. 7164, 37 FR 4190, Feb. 29, 1972; T.D. 74-13, 41 FR 12639, Mar. 26, 1976; T.D. 7528, 42 FR 64694, Dec. 28, 1977; T.D. 7728, 45 FR 72651, Nov. 3, 1980. Redesignated by T.D. 9354, 72 FR 45341, Aug. 14, 2007]