That means they knew the target date and +- time.
With 8 hours time difference (flight time), we're likely into 8-11 in Afghanistan.
Sh**, it almost sounds like they picked them up on their way to the airport or nearby motel.
This plot was orginally scheduled for December 2003. It's in the FR Threat Matrix archives and the plot to bring down all the airplanes was known on the Jihadi boards at the time as "Jihad Falcon" by the infamous terror propagandist EOM (aka Adam Gadahn).
http://www.freerepublic.com/focus/f-chat/1013726/posts?page=2584#2584
"U.S. raises terrorism threat level - Ridge: Possible attacks could rival 9/11"
CNN.Com / U.S. ^ | Sunday, December 21, 2003 Posted: 4:10 PM EST (2110 GMT) | CNN.Com
Follow the archive thread and you can see how things played out at the time by both Al Qaeda and the Feds...
Some arrests in London,and High Whickham, (Thames Valley area near Heathrow.) I did not get the last place where arrests were made.