Rev. & Tax. Code, § 23042

(a) For taxable years beginning prior to January 1, 2000, and the first taxable year beginning on or after January 1, 2000, “income year” means:

(1) For the purposes of the tax imposed under Chapter 2 (commencing with Section 23101), the calendar year or the fiscal year upon the basis of which the net income is computed. “Income year” means, for the purposes of the tax imposed under Chapter 2 (commencing with Section 23101), in the case of a return made for a fractional part of a year, the period for which such return is made.

(2) For the purposes of the tax imposed under Chapter 1.5 (commencing with Section 23081), Chapter 3 (commencing with Section 23501), or Chapter 4 (commencing with Section 23701), wherever “income year” is used throughout this part, it means “taxable year” as that term is defined in Section 23041 for the purposes of the tax imposed under Chapter 1.5 (commencing with Section 23081), Chapter 3 (commencing with Section 23501), or Chapter 4 (commencing with Section 23701).

(3) For purposes of the tax imposed under Chapter 2.5 (commencing with Section 23400), the same as defined in subdivision (a) with respect to a taxpayer subject to the tax imposed under Chapter 2 (commencing with Section 23101) and the same as defined in subdivision (b) with respect to a taxpayer subject to the tax imposed under Chapter 3 (commencing with Section 23501) or Chapter 4 (commencing with Section 23701).

(b) For taxable years (other than the first taxable year) beginning on or after January 1, 2000, the term “income year” shall have the same meaning as the term “taxable year” (as defined by Section 23041).