[stingraycharles]

Mass, gravity and general relativity. It’s a fascinating and very counter-intuitive at some moments, such as this.

Same goes for people that stay on the moon, when they go back to earth their watches are slightly different than those on earth. GPS satellites need to be synced with earth clock periodically as well for this reason.

[bluedevil2k]

The GPS satellites have multiple general relativity effects on their definition of time. Since they're moving so fast, the internal clocks on the satellites move slower than Earth (about 7 microseconds a day), while the decreased gravity on the satellites causes the internal clocks to speed up compared to Earth (45 microseconds a day). Net effect is a speed up of 38 microseconds.

Also, the on-board chips are pre-programmed to take into account the relativistic effects and compensate when doing their calculations.