149. A Matter of Time
By Andrew D. Wright
There is an old expression that a man with a watch knows what time it is,
but a man with two watches is never sure. This applies to computers as
Your computer has a little clock at its heart, powered by a coin-size
battery or a charged capacitor. It's called the real-time clock and it
runs all the time, even when the computer is powered off.
When your computer is powered on, the computer's BIOS reads the time from
this clock then in turn passes this on to the computer's operating system
as it comes up.
The operating system is the only one of these computer time keepers that
is aware of time zones and daylight saving time. These get set by the user
when the operating system is first installed or updated through the Date
and Time option in Control Panel.
World time is governed by averaging out the times from some 300 highly
accurate atomic clocks in more than fifty locations around the
world. These clocks are constantly communicating by satellite with each
other. A complex formula taking into account the physical locations of
these clocks and their local conditions (elevation, ambient temperature,
variances in Earth's gravity, etc.) is used to produce what we think of as
Connected to these clocks are special computers called time servers. Time
servers are set up in layers. The first layer is directly connected to the
atomic clocks. Below them a second layer of time servers get their
information from polling a number of primary time servers and pass this on
down the line.
One of the earliest computer pioneers, a remarkable woman named Grace
Hopper who became a US Navy Rear Admiral at the age of 79 for her
contributions to information technology, used to give public lectures and
at them would hand out little pieces of string.
These pieces of string, about 28 cm or just under a foot long, she called
nanoseconds. This was the distance an electron could travel in one
nanosecond, or one-billionth of one second. She used this as a
demonstration for why there was lag on satellite communications and for
why very fast computers needed to be very small since even small distances
created delays in communications.
Since all of these ultra-accurate atomic clocks and time servers are miles
away from each other and all communications will have lag, the Network
Time Protocol or NTP was developed to take this into account.
All recent operating systems have NTP built into them. On Windows
machines, clicking to set the time will open a window with three tabs.
Select the Internet Time tab and you will see by default that the computer
will check its time against a server run by Microsoft called
The National Research Council of Canada is the keeper of Canadian time and
you can type in their server, time.nrc.ca into this window to use
it instead of the US west coast-based Microsoft server.
When you update your time, the delay in transmitting the time information
is calculated and the time on your computer is set so it and the remote
time server are in agreement.
The operating system then updates the BIOS clock and the real-time clock
on the motherboard so when the computer is turned off, it will keep the
Computers are good at many things but their internal time keeping tends to
be as accurate as a cheap digital watch, gaining or losing several seconds
a week. With version 4 of the Network Time Protocol, time signals over the
Internet are accurate to around 1/100 of a second.
National Research Council of Canada time information:
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Originally published 27 February 2009