CA Rev & Tax Code Section 32405

Interest shall be computed, allowed, and paid upon any overpayment of any amount of tax at the modified adjusted rate per month established pursuant to Section 6591.5, from the 16th day of the calendar month following the period during which the overpayment was made. 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 15th day of the calendar month following the date upon which the claimant, 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 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.