Just watched a program with Stephen Hawking about time-travel and it turns out that the further you get away from gravitation, the faster time goes. In his experiment there were 20 nanoseconds in difference on 2 atomic clocks after 24 hours with 1 clock on top of a 3800 meter high mountain and 1 on the ground. So if you spend a lot of your time in a plane, you might live shorter because time is faster up there. Pretty interesting, huh?