See Clock synchronization or Clock
In engineering and computing, clock drift refers to several related phenomena where a clock does not run at exactly the same rate as a reference clock. That is, after some time the clock "drifts apart" or gradually desynchronizes from the other clock. All clocks are subject to drift, causing eventual divergence unless resynchronized. In particular, the drift of crystal-based clocks used in computers requires some synchronization mechanism for any high-speed communication. Clock drift is most commonly expressed in parts-per million (ppm), in which each unit equates to 86.4 milliseconds of drift per day. Common market watches usually range from ± 1 to ± 100 ppm. More advanced clocks may even use parts-per billion, and the most accurate atomic clocks have reached an accuracy that is expressed in parts-per sextillion. Computer clock drift can be utilized to build random number generators. These can however be exploited by timing attacks.