Network Time Synchronization
Most people assume that computer clocks in servers, workstations and network devices are inherently accurate. This is incorrect. Most of these clocks are set by hand to within a minute or two of actual time and are rarely checked after that. Many of these clocks are maintained by a battery-backed, clock-calendar device that may drift as much as a second per day. Having any sort of meaningful time synchronization is impossible if such clocks are allowed to run on their own.
For More information:
http://www.endruntechnologies.com/network-time-synchronization.htm
