California Code, Revenue and Taxation Code - RTC § 23041

(a) For the purposes of the tax imposed under Chapter 2 (commencing with Section 23101), the calendar year, or the fiscal year for which the tax is payable.

(b) 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), the calendar year or the fiscal year upon the basis of which the net income is computed.

(c) For purposes of the tax imposed under Chapter 2.5 (commencing with Section 23400), (1) in the case of a taxpayer subject to the tax imposed under Chapter 2 (commencing with Section 23101), the calendar year or the fiscal year for which the tax is payable and (2) in the case of a taxpayer subject to the tax imposed under Chapter 3 (commencing with Section 23501) or Chapter 4 (commencing with Section 23701), the calendar or fiscal year upon the basis of which the net income is computed.

(d) For the purpose of the taxes imposed under this part, a period of 12 months or less.

(e) When referring to a calendar or fiscal year beginning before January 1, 2000, upon the basis of which the net income is computed, the term “taxable year” shall mean “income year,” as defined in subdivision (a) of Section 23042.