Cal. Rev. & Tax. Code § 19391

In any judgment of any court rendered for any overpayment, interest shall be allowed at the adjusted annual rate established pursuant to Section 19521 upon the amount of the overpayment, from the date of the payment or collection thereof to the date of allowance of credit on account of the judgment or to a date preceding the date of the refund warrant by not more than 30 days, the date to be determined by the Franchise Tax Board.