Time servers are like other computer servers in the sense they are usually located on a network. A time server gathers timing information, usually from an external hardware source and then synchronises the network to that time.
Often time servers are synchronised to a UTC (Coordinated Universal time) source which is the global standard time scale and allows computers all over the world to synchronised to exactly the same time. This has obvious importance in industries where exact timing is crucial such as the stock exchange or airline industry.
There are various sources that a time server can use as a timing reference. The Internet is an obvious source, however, internet timing references from the Internet such as nist.gov and windows.time can not be authenticated, leaving the time server and therefore the network vulnerable to security threats.
There are authenticated alternatives to the Internet, the most common being to use the GPS network. As the Global Positioning System is reliant on knowing exactly what time it is to ensure reliable location information, this information can be utilised by a time server.
A simple GPS antenna connected to the time server will allow the GPS timing reference to be regularly checked by the time server. A GPS time server will be accurate to within a few hundred nanoseconds (a nanosecond = a billionth of a second).
There are also a number of national radio broadcasts such as the WWVB signal from Colorado in the US, the MSF signal from Cumbria in the UK and the DCF-77 signal from Frankfurt in Germany.
These radio signals are limited in their range though and even in major cities such as London it can be difficult to receive a decent enough signal.
Most timing servers use NTP (Network Time protocol) there are other protocols available but NTP is predominately used and is thought of as the standard for timing protocols. NTp has been around for over 25 years and is currently on version 4 but is always being updated which is probably why it is by fat the most common timing protocol.
NTP time servers work within the TCP/IP suite and rely on UDP (User Datagram Protocol). A less complex form of NTP – Simple Network Time Protocol (SNTP) is used in some devices and applications where high accuracy timing is not as important and is also included as standard in Windows software (although more recent versions of Microsoft Windows have the full NTP installed and the source code is free and readily available on the Internet from ntp.org).