Cal. Code Regs. tit. 18 § 15961
If the tax is not paid on or before the original due date of the return (see Revenue and Taxation Code Section 15651), it becomes subject to interest at 7 percent per annum through December 31, 1975 and at the rate of 12 percent per annum thereafter until paid.
If the return is filed within the time specified by law, no interest is chargeable if payment is made with the return; interest is chargeable, however, as to any deficiency found to be due. If the tax is not paid on or before the original due date for filing the return, it begins to accrue interest as indicated in (a) above from such date until payment.
If the return is filed after the 15th day of the second month following the close of the calendar quarter in which the gift was made, any tax and penalty due becomes delinquent and subject to interest as indicated in (a) above from said date until paid. Interest accrues from the original due date of the return even though the Controller grants an extension of time beyond that date for filing the return. (See Section 15681).
In the case of a false or fraudulent return or in the case of a failure to file a return, any unpaid tax and penalty becomes delinquent on the 15th day of the second month following the close of the calendar quarter in which the gift was made, and thereafter bears interest as indicated in (a) above until paid.
In any case of a delinquency, the interest is computed upon the amount of delinquent tax plus any penalties accrued. (See Revenue and Taxation Code Section 15684. For penalties generally, see Section 15681 and Revenue and Taxation Code Sections 15681 through 15685).
NOTE: Reference: Section 15961, Revenue and Taxation Code.
Cal. Code Regs. Tit. 18, § 15961