Cal. Rev. & Tax. Code § 38422

In making a deterimination 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 underpayments and overpayments shall be computed in the manner set forth in Sections 38451 and 38606.