Originally Posted by LarryJ

That's not correct.

There are several metrics that are reported to the DOT including D:00, A:00, and A:14. A flight is considered on-time only if it arrives at the arrival gate within 14 minutes of the scheduled arrival time (A:14). Touching down or taking off have no bearing on the on-time reports.

D:00 means that the airplane began it's pushback no later than the scheduled departure time. In order to meet D:00, the door has to be closed, jetbridge moved, F/A arm & crosscheck complete and ensure that all passengers are seated, belted, and carry-on bags stowed. At that point they tell us that the cabin is prepared and close the cockpit door. Once that is done, and the ramp crew is ready, we call the ramp tower for pushback clearance. Often, we must wait a few minutes for traffic to clear before we are given our pushback clearance. Only then can the pushback begin and our OUT time be established. To depart on-time, D:00, the passenger door has to close several minutes before the scheduled departure time. That is why the boarding times and cutoffs are set as they are.

D:00 is only important internally as D:00 is the first building block for achieving the ultimate goal of A:00. The "on-time" stats from the DOT, and in the computer reservation systems, are not based on D:00, they are based on A:14.

United is expanding the technology to consider more factors than just D:00 when deciding whether or not to hold a flight for connecting passengers. The new IT is combining information on flight plan time, down-line connections, reroute options, crew time limitations, other flights holding for the gate, etc. and recommend when, and for how long, to hold flights. I don't deal with this directly so don't know the details of how it is implemented but it should lead to smarter go/hold decisions going forward.

To the OP, I'm sorry that you missed your connection. Operating flights on time, including the delayed inbound flight which ultimately caused your delay and misconnect, is a priority for all of us. Getting from your inbound flight to the departure gate so quickly is impressive. Most passengers would not have been able to make it that quickly which likely influenced the gate agent's decision to close the flight when he/she did. They have a screen in Aero which displays all the relevant information on inbound connecting passengers along with the arrival gate and flight status information. This gives them a pretty good idea of who can make the connection and who will not.