Anyone can get an accurate "time hack" by calling (202) 762-1401--this will get you the continuous announcement of precise time for the U.S. Naval Observatory Master Clock. One can also go to http://www.time.gov/ or, if you have a GPS receiver, simply use the time from the display to get a very accurate time reading.

Networking uses GPS time signals to maintain synchronization within some specified parameters (I don't know what they are). As a practical matter, there is some time added by the cycle of processing (at the source)-transmitting-receiving-processing (at the destination)--think of it as analogous to the amount of time it takes you to hear/see the exact time and then react by pushing in the crown on your watch to restart it. This adds some error, along with the error inherent to the timekeeping instrument itself, which accounts for why your atomic clocks are most likely only accurate to within +/- 2 seconds or so.

Bottom line is that no matter how accurate any time source is, there will always be some error present at the user end of the time stream. So despite your clicking the "Check-in" button on aa.com at what you see as exactly 24 hours before your flight, it's a crap-shoot as to whether your request will arrive at precisely the moment that aa.com "sees" the time as 24 hours before your flight. Try always sending your request at 23:59:55 before--that usually works out well for me.

YMMV, of course!

cheers!