West Virginia Code Chapter 11. Taxation § 11-21-4f. Effect of rate changes during taxable year

(a) If any rate of tax imposed by this article changes to become effective after the thirty-first day of December, of a calendar year, and if the taxable year includes the effective date of the change of rate (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 of rate, 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 months in each period bears to the number of months in the entire taxable year.

(b) For purposes of subsection (a):