A time server is a computer device that obtains a timing reference from a particular source and transfers it to a network and its clients.
A time server does not rely on a system clock for timing information (all though they can be set to) as the internal clocks in PCs are prone to drift. Instead a time server will request timing information from an authoritative source and synchronises to it.
Most time servers use NTP (network Time Protocol) to synchronise devices and networks using the information gathered by the time server.
Most network administrators prefer to get their timing reference from a hardware source, preferably one that relays UTC (Coordinated Universal Time) a global timescale based on the time told by atomic clocks.
Apart from Internet time sources, of which many are inaccurate and can’t be authenticated there are two other methods to receive UTC time: the GPS network and national time and frequency transmissions.
A GPS time server will receive the timing information via an roof-mounted antenna whilst a radio receiving time server will utilise the national broadcasts of the US (WWVB), the UK (MSF), Germany (DCF), France (TDF) and Japan (JJY).
These broadcasts can’t be received everywhere and are vulnerable to local geography and interference.
GPS receivers are available everywhere on the planet so long as the antenna has a clear view of the sky.
Some dedicated time servers can receive both radio and GPS signals and can utilise both as a timing source, providing even more accuracy.
A time server receiving UTC time through either method can realistically provide accuracy to within a few milliseconds (1/1000th of a second) and can perform even better over LANs/WANs.