A motion for a continuance shall be in writing and shall allege specifically the grounds upon which it is based and, when made by a defendant, must be verified by his affidavit or that of his counsel. It shall be filed at least seven days prior to the commencement of trial.

Upon written motion at any time and after contradictory hearing, the court may grant a continuance, but only upon a showing that such motion is in the interest of justice.