Cal. Rev. & Tax. Code § 46527

In any judgment, interest shall be allowed at the modified adjusted rate per annum established pursuant to Section 6591.5, 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 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 board.