[kstar]

More cool stuff about time - emphasis added in bold:

Atomic clocks, the Earth and relativity

Atomic clocks are very precise and have nearly no clock drift. The rotation of the Earth itself actually has much more clock drift (less accuracy) than modern atomic clocks. Thus to keep the Coordinated Universal Time (UTC) in line with the Earth's rotation some years a leap second has to be added.

As Einstein predicted, relativistic effects can also cause clock drift due to time dilation. This is because there is no fixed universal time, time being relative to the observer. Special relativity describes how two clocks held by people in different inertial frames (i.e. moving with respect to each other but not accelerating or decelerating) will each appear to tick more slowly to the other person.

In addition to this, general relativity gives us gravitational time dilation. Briefly, a clock in a higher gravitational field (e.g. closer to a planet) will appear to tick more slowly. People holding these clocks would agree on which clock appeared to be going faster.

Note that it is time itself rather than the function of the clock which is affected. Both effects have been experimentally observed.

Time dilation is of practical importance. For instance, the clocks in GPS satellites experience this effect due to the reduced gravity they experience (making their clocks appear to run more quickly than those on Earth) and must therefore incorporate relativistically corrected calculations when reporting locations to users. If special relativity were not accounted for, the reported location would be off by about 1 km.[1]

Source: http://en.wikipedia.org/wiki/Clock_drift

Best,

Kurt