A.R.S. § 43-921: Computation of tax when law changed

The tax for any period beginning in one calendar year ("first calendar year") and ending in the following calendar year ("second calendar year") where the law applicable to the computation of taxes for taxpayers reporting on a calendar year basis differs for the second calendar year from the law applicable to the first calendar year, shall, except as otherwise provided, be the sum of paragraphs 1 and 2:

1. The same proportion of a tax for the entire period, determined under the law applicable to the first calendar year and at the rates for such year, which the portion of such period falling within the first calendar year is of the entire period.

2. The same proportion of a tax for the entire period, determined under the law applicable to the second calendar year and at the rates for such year, which the portion of such period falling within the second calendar year is of the entire period.