NTP

The network time protocol serves to keep the internet’s clocks, date and day synchronized.

There are many internet time servers, these servers have access to atomic clocks, through their software these servers send the time and day to client machines through the UDP port 123. Since the number of clients is so enormous, NTP has a huge number of servers to provide for the requests. It is actually the algorithms in the program that adjust the settings in accordance with internet network transmission delays.

An NTP server can be setup on most OS, for instance on windows through the date and time option, the internet time tab will allow you to choose an NTP server and decide whether to leave the time synchronization on or off.