26 USC 15 - Effect of changes

(a) General rule 
If any rate of tax imposed by this chapter changes, and if the taxable year includes the effective date of the change (unless that date is the first day of the taxable year), then
(1) 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
(2) 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.
(b) Repeal of tax 
For purposes of subsection (a)

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(1) if a tax is repealed, the repeal shall be considered a change of rate; and
(2) the rate for the period after the repeal shall be zero.
(c) Effective date of change 
For purposes of subsections (a) and (b)
(1) 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
(2) 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.
(d) Section not to apply to inflation adjustments 
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).
(e) References to highest rate 
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.
(f) Rate reductions enacted by Economic Growth and Tax Relief Reconciliation Act of 2001 
This section shall not apply to any change in rates under subsection (i) of section 1 (relating to rate reductions after 2000).