When you're moving tasks in Gantt view, to reschedule/move around dates/tasks, the start date doesn't change. So for example, if I had a task "Work on Logo" that started at 9am on Friday and was due 12pm Friday, if I were to move that in Gantt view to Monday, rather than moving both dates, it just moves the due date, so it looks like I'm working on that task all day for like 4 days.