Cal. Rev. & Tax. Code § 12258

(a) (1) Any insurer or Medi-Cal managed care plan 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, 100 percent of the tax due for total operating revenue, as defined by Section 12241, that a Medi-Cal managed care plan receives for the service periods from January 1, 2009, to June 30, 2013, inclusive, not previously paid, but which continue to be subject to tax, shall be paid no later than 30 days after the effective date of this section. If a Medi-Cal managed care plan subsequently receives additional amounts includable in its total operating revenue for the service periods referenced in this section, 100 percent of the tax shall continue to be due and shall be paid no later than 30 days after receipt of those amounts.