A time server is a server computer that reads the actual time from a reference clock and distributes this information to its clients using a computer network. The time server may be a local network time server or an internet time server.
The most important and widely-used protocol for distributing and synchronising time over the Internet is the Network Time Protocol (NTP), though other less-popular or outdated time protocols continue in use. A variety of protocols are in common use for sending time signals over radio links and serial connections.
The time reference used by a time server could be another time server on the network or the Internet, a connected radio clock or an atomic clock. The most common true time source is a GPS or GPS master clock. Time servers are sometimes multi-purpose network servers, dedicated network servers, or dedicated devices. All a dedicated time server does is provide accurate time.
An existing network server (e.g. a file server) can become a time server with additional software. The NTP homepage provides a free and widely-used reference implementation of the NTP server and client for many popular operating systems. The other choice is a dedicated time server device.
The term "stratum" is used to label the closeness to a central or high quality time server. The stratum indicates the place of a particular time server in a hierarchy of servers. The scale is 0 to 14 where 0 is the most accurate and likely a highly specialized physical hardware device. Some time clients will reject a time update from a server whose stratum is too high, and most will prefer low strata time sources to higher ones. This can be a pitfall for administrators setting up an in-house time server with no true time source.
See also 
- Network Time Protocol
- Inter-range instrumentation group time codes (IRIG)
- AFNOR-NFS 87-500
- NENA 911
- IEEE 1344
- IEEE 1588
- NMEA 0183
- Official site of the Network Time Protocol (NTP)
- Pool of public NTP Time Servers
- Time server at the Open Directory Project
- For a comprehensive list of NTP Servers
- How to add to the list of available time servers in Windows
- A list of the Simple Network Time Protocol (SNTP) time servers that are available on the Internet
- NIST Internet time service
- NIST Internet Time Servers