California Code, Revenue and Taxation Code - RTC § 19340

(a) In the case of a credit, from the date of the overpayment to the due date of the amount for which the credit is allowed. Any interest allowed on any credit shall first be credited on any amounts due from the taxpayer under Part 10 (commencing with Section 17001), this part, or Part 11 (commencing with Section 23001).

(b) In the case of a refund, including a refund in excess of tax liability as prescribed in subdivision (j) of Section 17053.5, from the date of the overpayment 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.