At any time more than fifteen days before the trial begins, a party defending against a claim may serve upon the adverse party an offer to allow judgment to be taken against him for the money or property or to the effect specified in his offer, with costs then accrued. If within ten days after the service of the offer the adverse party serves written notice that the offer is accepted, either party may then file the offer and notice of acceptance together with proof of service thereof and thereupon the court shall enter judgment. An offer not accepted shall be deemed withdrawn and evidence thereof is not admissible except in a proceeding to determine costs. If the judgment finally obtained by the offeree is not more favorable than the offer, the offeree must pay the cost incurred after the making of the offer. The fact that an offer is made but not accepted does not preclude a subsequent offer. When the liability of one party to another has been determined by verdict, order or judgment, but the amount or extent of the liabilityremains to be determined by further proceedings, the party adjudged liable may make an offer of judgment, which shall have the same effect as an offer made before trial if it is served within a reasonable time, not less than ten days, prior to the commencement of hearing to determine the amount or extent of liability.