In particular, the drift of crystal-based clocks used in computers requires some synchronization mechanism for any high-speed communication.
The rate of drift depends on the clock's quality, sometimes the stability of the power source, the ambient temperature, and other subtle environmental variables.
A quartz oscillator is less subject to drift due to manufacturing variances than the pendulum in a mechanical clock.
As Einstein predicted, relativistic effects can also cause clock drift due to time dilation.
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 general relativity were not accounted for, a navigational fix based on the GPS satellites would be false after only 2 minutes, and errors in global positions would continue to accumulate at a rate of about 10 kilometers each day.
If the OS timer and the CPU run on two independent clock crystals the situation is ideal and more or less the same as the previous example.
In 2006, a side channel attack was published[5] that exploited clock skew based on CPU heating.
CPU heating is correlated with clock skew, which can be detected by observing timestamps (under the server's real identity).