How To Maintain Accurate Time On Your PC

By: David Evans

PC's have internal real time clocks designed to maintain system
time. The internal clocks rely on relatively low-cost crystal
oscillators, which are notoriously inaccurate. In many cases PC
system time can drift by several minutes each day. This article
describes various methods of maintaining a highly accurate time
on your PC. It describes how to utilise Internet time
references, radio and GPS clocks and dedicated NTP server
systems.

Internet Time References

The Internet utilises the Network Time Protocol to distribute
accurate time information to network time clients. NTP has been
an important part of the Internet for over 25 years. The
protocol was born through the need to provide synchronisation of
time critical processes across the Internet.

Most modern operating systems, including Windows XP, 2003 and
LINUX have the ability to synchronise time with a NTP Server.
There are a host of Internet based NTP Servers with public
access that can be used to synchronise your PC. Below is a list
of popular Internet based public access NTP Server references:

time.nist.gov - US based time reference. ptbtime1.ptb.de -
German time reference. ntp.my-inbox.co.uk - UK time reference.
canon.inria.fr - French time reference. clock.isc.org - Canadian
time reference.

Windows XP has pre-installed SNTP client software that can
synchronise time with a NTP server. This is achieved by simply
entering the domain name of an Internet NTP Server in the time
properties tab. The host computer will then periodically contact
the NTP Server and synchronise the system time to the specified
reference.

LINUX and UNIX systems have the NTP software distribution
available from the NTP website, ntp.org. The NTP application
runs as a daemon, constantly monitoring specified NTP servers.
The application reads a list of NTP servers from a configuration
file, 'ntp.conf', and periodically synchronises time. A list of
NTP server references is specified with the 'server'
configuration command thus:

server time-a.nist.gov # NIST, Gaithersburg, Maryland NTP server
server time-c.timefreq.bldrdoc.gov # NIST, Boulder, Colorado NTP
server

Once configured, the NTP daemon can be started, stopped and
restarted using the commands: 'ntpd start'; 'ntpd stop' and
'ntpd restart'.

Radio and GPS Clock Timing References

There are many commercially available radio and GPS clock timing
references for PC's. Radio and GPS clocks obtain accurate time
from national radio timing references or from the GPS system.
The devices have serial or USB interfaces and provide an
accurate timing reference to PC's. Driver software periodically
synchronises the system time of the host computer to the
received reference time. Often reference clock drivers are
available that interface to the NTP application software to
provide your own dedicated NTP server.

Radio time receivers often have the advantage of having the
ability to obtain a good signal, indoors, close to the host
computer. However, the signal can only be received locally to
the transmitter. There are a number of radio time broadcasts
available: MSF transmitted from Rugby, UK; DCF-77 transmitted
from Frankfurt, Germany; WWVB transmitted from Colorado, USA;
CHU, Canada and TDF, France. GPS has the advantage of providing
a global time and frequency solution; however, an externally
located antenna is required which can be impractical.

Dedicated NTP Servers

NTP servers are dedicated time servers that obtain time from an
external source, such as radio or GPS, and provide a network
with an accurate timing reference. NTP servers are generally
rack-mountable devices with an external antenna and an Ethernet
connection. The devices obtain time from a radio or GPS timing
reference and maintain an accurate internal time. The accurate
time is then distributed to network time clients over an IP
network. Dedicated NTP servers often minimize the set-up and
configuration effort required to get a NTP server installation
up and running.

About the author:
Dave Evans develops NTP Server synchronisation systems to ensure
accurate time on PC's and computer networks. Dave has been
involved in the development of dedicated NTP server systems, NTP
synchronised digital wall clock systems and atomic clock time
synchronisation products. Find out more about SNTP and NTP
Server Systems at:
http://www.timetools.co.uk/ntp-servers/ntp-server-s5000.htm