That's an interesting concept... I hadn't thought about that before now! But my only concern would be that affixing a more concrete clock might permanently inconvenience trades with people halfway around the world if cars were had to be traded by a single time zone, wouldn't it?
The Race Clock is just the server's time (it's on EST). I didn't feel like writing a more complex script to convert that time to GMT as it would have to accomodate for DST. The original point of the race clock was to provide a synchronized time for joining GT5 Prologue online races, so only minutes/seconds really mattered.(Is there any reason why the race clock isn't GMT, or is it just because of the large number of US users?)