PC's keep time, even when the computer is turned off, using battery-backed real time clock (RTC) chips. The RTC chips are mass-produced low-cost components that generally do not have good time keeping properties and have a tendency to drift. This article discusses a number of ways to maintain accurate time on a PC. It also provides information on how to synchronise a PC's time with precise Internet and national time and frequency radio transmissions.

Computer systems time can drift anything from a few seconds a day to a few minutes each day. When processing transactions or performing time critical tasks across a number of computers, incorrect time can be a real headache.

The Internet found and solved this problem a number of years ago by developing the Network Time Protocol. NTP can be used to distribute accurate time from a highly precise time server to network time clients. Most modern operating systems have the ability to synchronise time with a NTP server. Generally, all that is required is the IP address or domain name of Stratum 1 or Stratum 2 NTP servers.

LINUX and UNIX operating systems can download the full NTP implementation from the NTP web site 'ntp.org'. NTP is freely available open source software available under the GNU public licence.

Microsoft Windows XP/2000/2003 and Vista operating systems have an integrated SNTP client. SNTP stands for Simple Network Time Protocol. SNTP is a sub-set of the Network Time Protocol. SNTP does not include many of the complex algorithms available to NTP to filter time and maintain high precision. Windows operating systems allow an IP address or domain name of a Internet or Intranet NTP server to be entered in the time properties tab. The SNTP client will then periodically contact the NTP server in order to update and synchronise system time.

A problem arises, however, if the computer does not have access to the Internet or is not networked. What is required is local access to an accurate time reference. Luckily, a number of precise freely available timing references are available to the public.

A number of national time and frequency radio broadcasts are available. These radio broadcasts are generally referred to by their 'call sign'. WWVB is the North-American time and frequency radio transmission, broadcast for Colorado in the States. DCF is the call-sign of the German radio time transmission, which is broadcast from Meinflingen near Frankfurt. The UK time transmitter is located at Anthorn, Cumbria, it's call sign is MSF. A number of other broadcasts are available in France, Canada, Switzerland and Japan. With the addition of a small RS232 serial or USB radio receiver, a PC can obtain continuous accurate time. The PC system time can then be synchronised to the received time and frequency radio source.

The problem with national radio time broadcasts is that they inherantly have a finite transmission range. Their transmissions are generally regional and limited to national boundaries. The Global Positioning System (GPS) provides a solution to this. The GPS system operates from a number of orbiting satellites. Each satellite has an on-board highly accurate synchronised atomic clock. GPS can provide highly precise timing information anywhere on the face of the planet. All that is required to receive the GPS transmissions is a low-cost GPS receiver and antenna. The GPS antenna needs to be shown a good clear view of the sky for correct operation. By connecting a PC to a serial or USB GPS receiver, accurate timing information is continuously available.

To summarise, a wide range of highly accurate computer timing references are freely available. Utilising the Internet, National time and frequency radio transmissions or the GPS system can ensure that a computers time remains locked to a precise time reference.