When any real estate has failed to be assessed for ad valorem taxes for any prior year or years, the same shall be assessed for ad valorem taxes for said prior year or years by the county assessor, and the taxes thereupon may be paid without the payment of any penalty or interest accruing prior to the date of assessment, provided that all taxes are paid within thirty (30) days after the date of such assessment and the sending of written notice thereof. If not so paid within said thirty (30) days, it shall be the duty of the county treasurer to collect the same in the manner provided by law, together with penalty at the lawful rate calculated from the date the same would have been delinquent had it been timely assessed, but in no event to an extent greater than one hundred percent (100%) of the principal amount thereof and not to exceed fifteen (15) years.