Atomic clocks provide extremely precise time. They are accurate to within 1 second every million years. However, they are extremely complex and expensive pieces of equipment that are only found in national standards laboratories. Genuine atomic clocks are not a viable option for synchronising computers and computer networks. There are, however, alternative sources of highly precise time.

The Global Positioning System (GPS) uses very precise timing to triangulate positioning information around the globe. Each of the 24 orbiting GPS satellites has an atomic clock onboard, which is synchronised by the US Naval Observatory (USNO). This timing information is made available to anyone with a relatively low-cost GPS receiver. Therefore, by interfacing a GPS receiver to a computer provides a highly accurate source of time.

There are also a number of national time-and-frequency radio broadcasts available around the world. These transmissions consist of time and date information that can be synchronised to an accurate pulse reference. The radio broadcasts are only available within a certain range of the transmitter - usually around 1500 - 2000 km. Using inexpensive radio receiving equipment, these broadcasts can easily be used to synchronise time on a computer or network.

Radio time and frequency broadcasts are available is most developed countries. Europe has the DCF-77 and MSF time signal, based in Germany and the UK respectively. The WWVB signal covers most of the USA. JJY is broadcast from Japan. Time broadcasts are also available in France, Canada and Switzerland.

NTP Time Servers are timing devices that utilise GPS and radio time broadcasts to provide an accurate time reference to computers and networks. Devices are available from a number of manufacturers that maintain accurate time and can synchronise many thousands of clients over an IP network.

Probably the most cost-effective way of getting atomic time on your computer is to synchronise to an Internet based NTP Time Server. There are many of these servers available, based in numerous countries. They are generally subscription free services. Internet based time reference do however have a number of disadvantages over locally based servers. Firstly, the network must have access to the Internet. Secondly, there are security issues; a port must be left open on the network firewall. Thirdly, many Internet based NTP servers may not be trustworthy.

Ultimately, an in-house NTP Time Server provides the most accurate, secure and reliable source of time for a computer network. Using any one of a number of precision time references available. In the long run, it may well provide the most cost-effective solution to computer time synchronisation.