The township engineer shall certify to the township secretary the date of commencement and of completion of all municipal improvements, the cost of which, in whole or in part, is to be paid by the owners of the abutting property. The certification shall be made a part of the permanent records of the township. The certified time of commencement and completion is conclusive on all parties. The term “certified time of completion” means the time of the completion of the whole contract for the improvement.