California Code, Revenue and Taxation Code - RTC § 13550

(a) The tax imposed by this part does not bear interest if it is paid prior to the date on which it otherwise becomes delinquent. However, if the tax is paid after that date, the tax bears interest at the rate for underpayment of estate tax provided in Section 6621(a)(2) of the Internal Revenue Codefrom the date it became delinquent and until it is paid.Interest under this section shall be compounded daily.

(b) The amendments made by Chapter 323 of the Statutes of 1998 shall apply to delinquent amounts unpaid on or after January 1, 1999, to December 31, 2000, inclusive.