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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 well.

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 standard time.

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, 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 new time.

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


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