California Code, Revenue and Taxation Code - RTC § 12258

(a) Any insurer that fails to pay any prepayment within the time required shall pay a penalty of 10 percent of the amount of the required prepayment, plus interest at the modified adjusted rate per month, or fraction thereof, established pursuant to Section 6591.5, from the due date of the prepayment until the date of payment but not for any period after the due date of the annual tax. Assessments of prepayment deficiencies may be made in the manner provided by deficiency assessments of the annual tax.

(b) Notwithstanding any other law, if a Medi-Cal managed care plan, as defined in subdivision (a) of Section 12009, receives additional amounts includable in its total operating revenue, as defined in Section 12241, for the service periods from January 1, 2009, to June 30, 2013, inclusive, those amounts shall continue to be subject to the tax imposed by Section 12201, as added by Section 4 of Chapter 33 of the Statutes of 2013, as added by Section 5 of Chapter 157 of the Statutes of 2009, as added by Section 31 of Chapter 717 of the Statutes of 2010, and as added by Section 2 of Chapter 11 of the First Extraordinary Session of the Statutes of 2011, and 100 percent of the tax continues to be due and shall be submitted to the Department of Insurance no later than 30 days after receipt of those amounts.

(c) This section does not apply to an insurer subject to paragraph (1) of subdivision (c) of Section 12254.