California Code, Revenue and Taxation Code - RTC § 12302

(a) On or before April 1 (or June 15 with respect to taxes on ocean marine insurance) every person that is subject to any tax imposed by Section 28 of Article XIII of the California Constitution or this part, in respect to the preceding calendar year shall file, in duplicate, an insurance tax return with the commissioner in the form as the commissioner may prescribe. The return shall show that information pertaining to its insurance business in this state as will reflect the basis of its tax as set forth in Chapter 2 (commencing with Section 12071) and Chapter 3 (commencing with Section 12201) of this part, the computation of the amount of tax for the period covered by the return, the total amount of any tax prepayments made pursuant to Article 5 (commencing with Section 12251) of Chapter 3 of this part, and any other information as the commissioner may require to carry out the purposes of this part. Separate returns shall be filed with respect to the following kinds of insurance:

(b) This section shall become operative on July 1, 2013.