(1) The offeror may state in the offer that it is open for acceptance until a specified date.
(2) The offer shall have no effect on any decision that the court makes as to the consequences of the offer unless it is open for acceptance for at least 21 days.
(3) Acceptance of the offer after the beginning of the trial shall have no effect on any decision that the court makes as to the consequences of such acceptance.
(4) The court may permit an offeree to accept an offer after the specified date on such terms as the court considers just.