California Code, Revenue and Taxation Code - RTC § 12631

(a) Any insurer that fails to pay any tax, except a tax determined as a deficiency assessment by the board under Article 3 (commencing with Section 12421) of Chapter 4, within the time required, shall pay a penalty of 10 percent of the amount of the tax in addition to the tax, plus interest at the modified adjusted rate per month, or fraction thereof, established pursuant to Section 6591.5, from the due date of the tax until the date of payment.

(b) This section shall become operative on July 1, 2013.