Unfortunately all the common clock hardware is not very accurate. This is simply because the frequency that makes time increase is never exactly right. Real clocks have a frequency error of several PPM quite frequently. Even if the systematic error of some clock model is known, the clock will never be perfect. This is because the frequency varies over time, mostly influenced by temperature, but it could also be air pressure or magnetic fields, etc.
Reliability determines the time a clock can keep the time within a specified accuracy.
For long-term observation one may also notice variations in the clock frequency. The difference of the frequency is called wander . As explained before, it's not sufficient to correct the clock once. To illustrate the problem, have a look at Figure 1 . The offset of a precision reference pulse has been measured with the free-running system clock.