From the History Commons Groups blog:

Many new entries have been added to the Complete 9/11 Timeline at History Commons, most of them describing events that took place on the day of 9/11.

Two important entries deal with events before 9/11, specifically regarding military training exercises. One entry reveals that prior to 9/11, NORAD practiced for hijackings five times per month at its operations center in Colorado. The other entry describes how, in the days before 9/11, the Army had been planning an exercise for the week after 9/11, based, remarkably, on the scenario of a plane crashing into the World Trade Center.

Several entries give new details about the actions of the two F-15 fighter jets launched in response to the first hijacked plane. After being told by the military that the fighters had been given a "bad heading," an air traffic controller had to redirect the two aircraft. When he did this, he also notified the lead fighter pilot that Flight 11 had crashed into the WTC. And yet the two pilots have both denied learning this news until about 10 minutes later, when they were told that a second plane had hit the WTC. After being given the new heading, the lead pilot contacted the military, but apparently neither he nor the person he spoke with mentioned the crash during their call. The two fighters then headed into military airspace over the ocean.

A number of entries deal with the actions of the US Park Police Aviation Unit in Washington, DC, on September 11. That morning, the unit had been holding a large training event. Its two Huey helicopters promptly responded to the Pentagon attack, with one of them providing live video footage of the crash scene and also being put in charge of controlling the Washington airspace. However, its pilot's request that the Maryland State Police send helicopters to help out at the Pentagon was bluntly refused. The other Park Police helicopter subsequently had to respond to numerous reported emergencies that turned out to be false alarms. Furthermore, contrary to official accounts, a new timeline entry reveals that one of the Park Police helicopters was in the air before the Pentagon attack and was instructed to intercept the approaching aircraft.

A US Border Patrol agent who saw the helicopter from his hotel window also described seeing an American Airlines plane taking off from Washington's Reagan National Airport, flying north toward the nearby Pentagon, about a minute before the Pentagon attack occurred. And several witnesses have described hearing secondary explosions inside the Pentagon following the attack there.

An entry describes how National Security Adviser Condoleezza Rice thought the first plane hitting the WTC was an accident when she was informed of it by her assistant. While phoning President Bush about the incident, Rice was told that the crashed plane was a commercial airliner, and she passed on this news to the president. Rice has also described how those in the shelter conference room below the White House were unable to hear the audio from television broadcasts and the White House video teleconference at the same time throughout the day, due to technical problems.

Unlike Rice, those in the office of the vice chief of naval operations at the Pentagon realized right away that the first crash must have been deliberate, and even speculated that the Pentagon was a likely target for any subsequent attack. But when a senior officer there was told of a suspicious aircraft approaching Washington, he instructed his deputy to keep quiet and not pass on this crucial information.

Some entries describe how United Airlines, as well as having two of its aircraft hijacked that morning, had to deal with numerous false alarms. These included threats that were found to be "misunderstandings or hoaxes"; a loss of contact with three aircraft at around 10:00 a.m.; nine aircraft reported missing about an hour later; and another of its aircraft reported as missing by the Secret Service.

Later in the day, those in the White House were told that a US Airways plane flying to the US from Spain had been hijacked, but this also turned out to be a false alarm.

Source.