The chairman of the board of supervisors is hereby authorized, in the name of the county to execute the contract with the awardee thereof, and to receive and approve all bonds required by this act on the part of the awardee, and shall, by the terms of said contract, fix the time for the beginning of the work, which shall not be more than 20 days from the date thereof. The contract shall provide that the work shall be prosecuted with diligence until completed, and shall fix a time for such completion. Such time of completion may be extended from time to time by the board of supervisors, in its

discretion, and by resolution, entered by the clerk in the minutes of said board, a copy of which shall by said clerk be endorsed upon or annexed to the contract.