[azstar]

When an inbound aircraft leaves its origin city late, computers automatically update everything with an "estimated" departure time for the outbound flight because passengers insist on being informed. However, frequently the aircraft arrives much earlier than the expected time and departs earlier than the "estimated" posted time. Unfortunately, many infrequent travelers believe the posted departure time to be official, and they show up, thinking they're fine, to an empty gate and a departed aircraft. That is one of the reasons that every "contract of carriage" states you must be at the gate 20-30 minutes prior to departure.