Cal. Rev. & Tax. Code § 506

The tax rate applicable to any assessment made pursuant to this article shall be the tax rate to which the property would have been subject if it appeared upon the roll in the year when it should have been lawfully assessed. To the tax there shall be added interest at the rate of three-fourths of 1 percent per month from the date or dates the taxes would have become delinquent if they had been timely assessed to the date the additional assessment is added to the assessment roll.