A time server is computer hardware that performs the function of obtaining accurate time from a timing reference and transfers this time to network infrastructure and other clients. There are many time servers that reside on the Internet and serve accurate time. However, many organisations have their own independent time servers located on their local network, or intranet.

A time server receives accurate time from an external timing reference to maintain precise internal time. There are a number of hardware clocks that a time server may utilise to obtain accurate time. One of the most common external hardware clock references is the Global Positioning System, or GPS. GPS satellites provide highly accurate timing and positioning information. The timing component of GPS transmissions is accurate to within a couple of hundred nanoseconds of the correct time. There is also a number of National time and frequency radio broadcasts available. The WWVB signal is transmitted from Colorado and is available throughout the USA. The DCF-77 radio time signal is broadcast from Frankfurt, Germany, and is available throughout much of Central and Western Europe. While, the MSF-60 radio time signal is transmitted from Anthorn in Cumbria, UK, and can be received throughout much of North-Western Europe. Alternatively, a time server may just obtain time from another time server in a hierarchy of time servers.

A time server may be an existing file server that also provides timing information in addition to it's primary function as a file server. Alternatively, it may be a dedicated device whose only function is to provide time to client computers on the network. The advantage of dedicated devices is that additional hardware may be incorporated to maintain accurate time in the event of external reference clock signal loss. Typically, many dedicated time servers have back-up TCXO, OCXO or Rubidium crystals that are much more accurate than standard quartz crystals. A Rubidium crystal may typically lose only a few milliseconds per year.

There are a number of protocols that can be used for precise time transfer between computers. Many of these protocols have become obsolete. The most common protocol used for time transfer between computers is NTP - the Network Time Protocol. The Network Time Protocol is one of the oldest protocols still in use today. It was developed over 25 years ago as a means of transfering accurate time around the internet. The NTP protocol is descibed in a detailed RFC and defines methods and data structures for sharing accurate time between computers on a network. NTP operates in a hierarchical manner, with a level 1, or stratum 1, time server sitting at the top of the tree and filtering time information to lower stratum time servers. The lower the stratum of the time server, the further away it is from the reference server and hence less accurate.