California Code, Revenue and Taxation Code - RTC § 19066

(a) For the purposes of Sections 19057, 19058, and 19065, a return of tax imposed under Part 10 (commencing with Section 17001) or Part 11 (commencing with Section 23001) except a return required by Article 5 (commencing with Section 18661) of Chapter 2 (relating to withholding), filed before the last day prescribed by law for filing (determined without regard to any extension of time for filing the return), shall be considered as filed on that day. For purposes of Section 19306, payment of any portion of the tax made before the last day prescribed for the payment of the tax shall be considered made on the last day.

(b) For purposes of this section, if a return required by Article 5 (commencing with Section 18661) of Chapter 2 (relating to withholding) or a return of tax imposed by Section 13020 of the Unemployment Insurance Code (relating to withholding tax on wages), for any period ending with or within a calendar year is filed before April 15 of the succeeding calendar year, that return shall be considered filed on April 15 of that calendar year.