All warrants issued to any contractor shall be redeemed out of the special fund of such local improvement district upon call by the city treasurer and shall bear interest at the rate of not to exceed eight percent per annum from their date until the date of maturity provided therein, after which they shall not draw any interest; that the date of maturity provided in such warrants shall be a date sixty (60) days after the date provided for the completion of the contract as set out in the specifications and contract.