Also, 1.5 hours only exists for things like fire doors and fire dampers. For actual fire separations, it is 0hr, 45min, 1hr, 2hr, 4hr.
0hr sounds like a weird one, but it is intended as a smoke control only. Fire separations are barriers that control the spread of fire and smoke. A fire resistance rating is the time in minutes or hours a particular assembly will resist the passage of flame and is assigned to a fire separation.