The assessment roll shall include all items of cost provided by law, and in the event that warrants or other evidence of indebtedness drawing interest against such district are issued previous to the time when assessments are payable as provided by law, such roll shall include interest at the rate for such warrants or indebtedness computed on the amount of such interest-bearing indebtedness to a period 30 days after the date for hearing on the roll, and in the event that it is proposed to issue local improvement bonds such roll shall include such interest for an additional period of 20 days; provided, that persons paying assessments within the 30-day period within which no interest, penalty or costs are charged shall be entitled to a deduction of interest not accrued as aforesaid. The object of the above provision is to assure enough to the local improvement fund to pay the bonds, with interest, and full payment of warrants. (1957 code § 8.02.050.)