69(4)An award under this section may be retroactive to the date of appointment of a conciliation officer or mediator, or to such earlier or later date as may be fixed in the award of the conciliation board, as the case may be, but in no case shall the award be retroactive, where no collective agreement was in operation, to a day before the day on which notice to bargain collectively was given by either party, or, where a collective agreement was in operation, before the expiration date of the agreement or the expiration date of a provision therein subject to revision under the agreement.