Streets and Highways Code § 18098

When payment of any assessment is made, the tax collector shall mark opposite such assessment the word “paid,” with the date of the payment thereof, and shall give a receipt therefor. Upon the expiration of said period of 30 days, all assessments then unpaid shall become delinquent, and the tax collector shall mark each such assessment “delinquent,” and shall add 5 per cent to the amount thereof.