California Code, Revenue and Taxation Code - RTC § 12257

(a) If the total amount of prepayments for any calendar year exceeds the amount of annual tax for that year, the excess shall be treated as an overpayment of annual tax and, at the election of the insurer, may be credited against the amounts due and payable for the first prepayment of the following year. Any amount of the overpayment not so credited shall be allowed as a credit or refund under Article 2 (commencing with Section 12977) of Chapter 7 of this part.

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