Cal. Sts. & High. Code § 8681

Interest on all unpaid assessments shall begin to run from the date of filing of the contractor’s statement, or as otherwise provided in the law pursuant to which the assessment was levied, or from the date of the bonds if the assessment was levied under the Municipal Improvement Act of 1913 or other law, and shall be computed at the rate specified in the bonds secured by the assessments. For each year interest shall be computed and collected up to the next second day of September succeeding, no deduction being made by reason of any installment of the assessment being due or paid prior thereto in such year.