Cal. Rev. & Tax. Code § 7714

In making a determination the board may offset overpayments for a period or periods, together with interest on the overpayments, against underpayments for another period or periods, against penalties, and against the interest on the underpayments. The interest on the underpayments and overpayments shall be computed in the manner set forth in Sections 7674 and 8130.