California Code, Revenue and Taxation Code - RTC § 19306

(a) No credit or refund shall be allowed or made after a period ending four years from the date the return was filed (if filed within the time prescribed by Section 18567 or 18604, whichever is applicable), four years from the last day prescribed for filing the return (determined without regard to any extension of time for filing the return), or after one year from the date of the overpayment, whichever period expires later, unless before the expiration of that period a claim therefor is filed by the taxpayer, or unless before the expiration of that period the Franchise Tax Board allows a credit, makes a refund, or mails a notice of proposed overpayment on a preprinted form prescribed by the Franchise Tax Board.

(b) The amendments to this section by the act adding this subdivision  1 shall be applied to all claims and refunds, without regard to taxable year, for which the statute of limitations has not expired on the date that this act takes effect.