Township officers are entitled to compensation at the rates specified in this Article for each day necessarily devoted by them to the services of the township in the duties of their respective offices. Compensation set by a multi-township board for the multi-township assessor shall be set at least 150 days before the election of that officer. Compensation set by a township board for the township assessor and collector shall be set at the same time the compensation of its supervisor is set. Compensation shall be for time served, and no township officer may receive compensation for any future or anticipated days of duty.