Cal. Revenue and Taxation Code § 12983.1

Interest shall be allowed upon the amount of any overpayment of tax by a surplus line broker pursuant to this part at the rate of 1 percent per calendar month or fraction thereof, from the first day of the monthly period following the period during which the overpayment was made. For purposes of this section, “monthly period” means the month commencing on the day after the due date of the payment through the same date as the due date in each successive month. In addition, a refund or credit shall be made of any interest imposed upon the claimant with respect to the amount being refunded or credited. The interest shall be paid as follows: (a) In the case of a refund, to the last day of the calendar month following the date upon which the claimant is notified in writing 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.