Cal. Revenue and Taxation Code § 24251

The tax on any taxpayer for a period beginning in one calendar year (called “first calendar year”) and ending in the following calendar year (called “second calendar year”) where the law applicable to the computation of taxes for calendar year taxpayers for the second calendar year is different from the law applicable to computation of taxes for calendar year taxpayers for the first calendar year, shall, except as otherwise provided, be the sum of: (a) The same proportion of a tax for the entire period, determined under the law and rates applicable to the first calendar year which the portion of the period falling within the first year is of the entire period; and (b) The same proportion of a tax for the entire period, determined under the law and rates applicable to the second calendar year which the portion of the period falling within the second calendar year is of the entire period. Any tax that has been paid under the law applicable to the first calendar year if in excess of the tax imposed by this section shall be refunded or credited to the taxpayer as provided in Chapter 22. Any tax in addition to that paid under the law applicable to the first calendar year made necessary by this article shall be immediately due and payable upon notice and demand from the Franchise Tax Board.