Most people realize that, for an airline, an "on-time departure" means that the plane pushed back from the gate within 15 minutes of their scheduled departure time. Of course, the airplane may sit on the tarmac for who knows how long.
Enter one of the 'Net's best kept secrets, flightaware.com, which is great at displaying realtime aircraft tracking from the FAA.
UAL reports that this flight I was tracking (UAL 5744) departed seven minutes ahead of schedule. So, what time do I really need to get to the airport to pick up my wife?
Well, it turns out that the flight went wheels-up almost 25 minutes late, but it still arrived ahead of schedule.
Not bad - in reality, this flight typically takes 25 minutes from takeoff to landing.