210 lines
4.9 KiB
HTML
210 lines
4.9 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
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<HTML
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><HEAD
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><TITLE
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>The concept of localtime</TITLE
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><META
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NAME="GENERATOR"
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CONTENT="Modular DocBook HTML Stylesheet Version 1.7"><LINK
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REL="HOME"
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TITLE="Linux System Administrators Guide"
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HREF="index.html"><LINK
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REL="UP"
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TITLE="Keeping Time"
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HREF="keeping-time.html"><LINK
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REL="PREVIOUS"
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TITLE="Keeping Time"
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HREF="keeping-time.html"><LINK
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REL="NEXT"
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TITLE="The hardware and software clocks"
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HREF="hw-sw-clocks.html"></HEAD
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><BODY
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CLASS="SECT1"
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BGCOLOR="#FFFFFF"
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TEXT="#000000"
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LINK="#0000FF"
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VLINK="#840084"
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ALINK="#0000FF"
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><DIV
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CLASS="NAVHEADER"
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><TABLE
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SUMMARY="Header navigation table"
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WIDTH="100%"
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BORDER="0"
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CELLSPACING="0"
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><TR
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><TH
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COLSPAN="3"
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ALIGN="center"
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>Linux System Administrators Guide: </TH
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></TR
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><TR
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><TD
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WIDTH="10%"
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ALIGN="left"
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VALIGN="bottom"
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><A
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HREF="keeping-time.html"
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ACCESSKEY="P"
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>Prev</A
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></TD
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><TD
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WIDTH="80%"
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ALIGN="center"
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VALIGN="bottom"
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>Chapter 14. Keeping Time</TD
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><TD
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WIDTH="10%"
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ALIGN="right"
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VALIGN="bottom"
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><A
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HREF="hw-sw-clocks.html"
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ACCESSKEY="N"
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>Next</A
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></TD
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></TR
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></TABLE
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><HR
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ALIGN="LEFT"
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WIDTH="100%"></DIV
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><DIV
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CLASS="SECT1"
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><H1
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CLASS="SECT1"
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><A
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NAME="LOCALTIME"
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></A
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>14.1. The concept of localtime</H1
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><P
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> Time measurement is based on mostly regular natural
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phenomena, such as alternating light and dark periods caused
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by the rotation of the planet. The total time taken by two
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successive periods is constant, but the lengths of the light
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and dark period vary. One simple constant is noon. </P
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><P
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> Noon is the time of the day when the Sun is at its
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highest position. Since (according to recent research) the Earth is
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round, noon happens at different times in different places. This
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leads to the concept of <I
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CLASS="GLOSSTERM"
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>local time</I
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>. Humans
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measure time in many units, most of which are tied to natural
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phenomena like noon. As long as you stay in the same place,
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it doesn't matter that local times differ. </P
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><P
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> As soon as you need to communicate with distant places,
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you'll notice the need for a common time. In modern times,
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most of the places in the world communicate with most other
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places in the world, so a global standard for measuring time
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has been defined. This time is called <I
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CLASS="GLOSSTERM"
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>universal
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time</I
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> (UT or UTC, formerly known as Greenwich Mean Time
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or GMT, since it used to be local time in Greenwich, England).
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When people with different local times need to communicate,
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they can express times in universal time, so that there is no
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confusion about when things should happen. </P
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><P
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> Each local time is called a time zone. While geography
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would allow all places that have noon at the same time have the
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same time zone, politics makes it difficult. For various reasons,
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many countries use <I
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CLASS="GLOSSTERM"
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>daylight savings time</I
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>,
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that is, they move their clocks to have more natural light
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while they work, and then move the clocks back during winter.
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Other countries do not do this. Those that do, do not agree when
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the clocks should be moved, and they change the rules from year
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to year. This makes time zone conversions definitely non-trivial.
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</P
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><P
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> Time zones are best named by the location or by telling
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the difference between local and universal time. In the US
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and some other countries, the local time zones have a name and
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a three letter abbreviation. The abbreviations are not unique,
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however, and should not be used unless the country is also named.
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It is better to talk about the local time in, say, Helsinki,
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than about East European time, since not all countries in Eastern
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Europe follow the same rules. </P
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><P
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> Linux has a time zone package that knows about all
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existing time zones, and that can easily be updated when the
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rules change. All the system administrator needs to do is to
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select the appropriate time zone. Also, each user can set his
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own time zone; this is important since many people work with
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computers in different countries over the Internet. When the
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rules for daylight savings time change in your local time zone,
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make sure you'll upgrade at least that part of your Linux system.
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Other than setting the system time zone and upgrading the time
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zone data files, there is little need to bother about time.
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</P
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></DIV
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><DIV
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CLASS="NAVFOOTER"
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><HR
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ALIGN="LEFT"
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WIDTH="100%"><TABLE
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SUMMARY="Footer navigation table"
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BORDER="0"
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CELLPADDING="0"
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CELLSPACING="0"
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><TR
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><TD
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WIDTH="33%"
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ALIGN="left"
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VALIGN="top"
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><A
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HREF="keeping-time.html"
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ACCESSKEY="P"
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>Prev</A
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></TD
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><TD
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WIDTH="34%"
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ALIGN="center"
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VALIGN="top"
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><A
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HREF="index.html"
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ACCESSKEY="H"
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>Home</A
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></TD
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><TD
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WIDTH="33%"
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ALIGN="right"
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VALIGN="top"
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><A
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HREF="hw-sw-clocks.html"
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ACCESSKEY="N"
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>Next</A
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></TD
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></TR
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><TR
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><TD
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WIDTH="33%"
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ALIGN="left"
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VALIGN="top"
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>Keeping Time</TD
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><TD
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WIDTH="34%"
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ALIGN="center"
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VALIGN="top"
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><A
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HREF="keeping-time.html"
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ACCESSKEY="U"
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>Up</A
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></TD
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><TD
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WIDTH="33%"
|
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ALIGN="right"
|
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VALIGN="top"
|
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>The hardware and software clocks</TD
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></TR
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></TABLE
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></DIV
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></BODY
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></HTML
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> |