California Code, Revenue and Taxation Code - RTC § 19145

(a) The 15th day of the third month following the close of the taxable year, except in the case of an organization described in Section 23731 subject to the tax imposed under Section 23731, in which case “fifth” shall be substituted for “third.”

(b) With respect to any portion of the underpayment, the date on which that portion is paid. For purposes of this subdivision, a payment of estimated tax on any installment date shall be considered a payment of any previous underpayment only to the extent the payment exceeds the amount of the installment determined under subdivision (a) of Section 19144 for the installment date.