Construction work under any franchise granted, shall be commenced in good faith within not more than four months from the date of the taking effect of the ordinance granting such franchise, and if not so commenced within said time, said franchise shall be forfeited. Work under any franchise so granted shall be prosecuted with reasonable diligence to completion and shall be completed within the time fixed for such completion in the ordinance granting the same, and if not so completed within said time, the same shall be forfeited; provided, that the council may by resolution extend the time for the completion thereof as they may deem advisable.