(a) Time. A motion for new trial shall be filed not later than 15 days after return of verdict in a jury action or the date of filing of the judgment in a nonjury action. A timely motion may be amended to state new grounds at any time before it is disposed of in the discretion of the court.
(b) Determination. The motion shall set forth the basis with particularity. Upon examination of the motion, the court may find it without merit and deny it summarily, or may grant a hearing on it with notice.
(c) Grounds. All orders granting a new trial shall specify the specific grounds therefor. If such an order is appealed and does not state the specific grounds, the appellate court shall relinquish its jurisdiction to the trial court for entry of an order specifying the grounds for granting the new trial.