In a single system, there is one clock that provides the time of day to all applications. There is always one consistent time for all processes running on the system. Therefore, in a distributed environment consisting of multiple systems, each system operates by its own clock and each clock experiences clock error. System clock error tends to increase over time; the rate of change of error is known as drift. The clocks used in many computer systems have a specified maximum drift of several seconds per day.
If each system in a network started at the same time and ran at the same rate, the clocks would remain synchronized. Because each system clock drifts at a different rate, however, the clocks of systems in a distributed environment tend to become unsynchronized. The difference between any two clock readings is known as skew.
A distributed time service regulates system clocks to synchronize with each other and closely match a standardized time
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