Cal. Revenue and Taxation Code § 11555

Interest shall be paid upon the amount of any overpayment of tax at the adjusted annual rate established pursuant to Section 19521 upon the amount found to have been illegally collected from the date of payment of the amount to the date of allowance of credit on account of the overpayment or to a date preceding the date of the refund warrant by not more than 30 days, the date to be determined by the board from the first day of the month following the month the tax becomes due. The interest shall be paid: (a) In the case of a refund, to the last day of the calendar month following the date upon which the person making the overpayment, if he or she has not already filed a claim, is notified by the board that a claim may be filed or the date upon which the claim is approved by the board, whichever date is the earlier. (b) In the case of a credit, to the same date as that to which interest is computed on the tax or amount against which the credit is applied.