A time server is a computer server that reads the time from an accurate clock and distributes this information to its clients across a computer network.
The reference used by a time server is normally a UTC (coordinated universal time) time source. UTC is a global time scale adopted all over the world and based on the time told by atomic clocks The most common source for UTC time is now the GPS system (global positioning system).
The most widely-used protocol for distributing and synchronising time is Network Time Protocol (NTP) which has been around almost as long as the Internet itself, having been developed in 1985 by Professor David Mills.
NTP receives the time from the time server and then checks or clocks on its networks to see if they need advancing or retreat.
To prevent overload of networks requesting timing information, NTP is hierarchical. The term “stratum” is used to label the nearness to a stratum 0 server – that atomic clock. The higher the stratum number the further away the server is. A GPS time server is normally a stratum 1 device as it receives time from a stratum 0 device. However, stratum 2 and stratum 3 devices can still synchronise with each other.