26 U.S.C. § 15: Effect of changes

tentative taxes shall be computed by applying the rate for the period before the effective date of the change, and the rate for the period on and after such date, to the taxable income for the entire taxable year; and the tax for such taxable year shall be the sum of that proportion of each tentative tax which the number of days in each period bears to the number of days in the entire taxable year. if a tax is repealed, the repeal shall be considered a change of rate; and the rate for the period after the repeal shall be zero. if the rate changes for taxable years “beginning after” or “ending after” a certain date, the following day shall be considered the effective date of the change; and if a rate changes for taxable years “beginning on or after” a certain date, that date shall be considered the effective date of the change. This section shall not apply to any change in rates under subsection (f) of section 1 (relating to adjustments in tax tables so that inflation will not result in tax increases). If the change referred to in subsection (a) involves a change in the highest rate of tax imposed by section 1 or 11(b), any reference in this chapter to such highest rate (other than in a provision imposing a tax by reference to such rate) shall be treated as a reference to the weighted average of the highest rates before and after the change determined on the basis of the respective portions of the taxable year before the date of the change and on or after the date of the change. This section shall not apply to any change in rates under subsection (i) of section 1 (relating to rate reductions after 2000). Except as provided in paragraph (2), for purposes of section 15 of the Internal Revenue Code of 1986, no amendment or repeal made by this Act [see Tables for classification] shall be treated as a change in the rate of a tax imposed by chapter 1 of such Code. Paragraph (1) shall not apply to the amendment made by section 601 [amending section 11 of this title ] (relating to corporate rate reductions).”