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diff --git a/sag-0.6.1-www/sag-0.6.1.html/c2432.html b/sag-0.6.1-www/sag-0.6.1.html/c2432.html new file mode 100644 index 0000000..35c06e5 --- /dev/null +++ b/sag-0.6.1-www/sag-0.6.1.html/c2432.html @@ -0,0 +1,272 @@ +<!DOCTYPE HTML PUBLIC "-//Norman Walsh//DTD DocBook HTML 1.0//EN"> +<HTML +><HEAD +><TITLE +>Keeping Time</TITLE +><META +NAME="GENERATOR" +CONTENT="Modular DocBook HTML Stylesheet"><LINK +REL="HOME" +TITLE="The Linux System Administrators' Guide" +HREF="book1.html"><LINK +REL="PREVIOUS" +TITLE="Compressed backups" +HREF="x2417.html"><LINK +REL="NEXT" +TITLE="The hardware and software clocks" +HREF="x2451.html"></HEAD +><BODY +BGCOLOR="#FFFFFF" +TEXT="#000000" +><DIV +CLASS="NAVHEADER" +><TABLE +WIDTH="100%" +BORDER="0" +CELLPADDING="0" +CELLSPACING="0" +><TR +><TH +COLSPAN="3" +ALIGN="center" +>The Linux System Administrators' Guide</TH +></TR +><TR +><TD +WIDTH="10%" +ALIGN="left" +VALIGN="bottom" +><A +HREF="x2417.html" +>Prev</A +></TD +><TD +WIDTH="80%" +ALIGN="center" +VALIGN="bottom" +></TD +><TD +WIDTH="10%" +ALIGN="right" +VALIGN="bottom" +><A +HREF="x2451.html" +>Next</A +></TD +></TR +></TABLE +><HR +ALIGN="LEFT" +WIDTH="100%"></DIV +><DIV +CLASS="CHAPTER" +><H1 +><A +NAME="AEN2432" +>Chapter 11. Keeping Time</A +></H1 +><DIV +CLASS="TOC" +><DL +><DT +><B +>Table of Contents</B +></DT +><DT +><A +HREF="c2432.html#AEN2438" +>Time zones</A +></DT +><DT +><A +HREF="x2451.html" +>The hardware and software clocks</A +></DT +><DT +><A +HREF="x2457.html" +>Showing and setting time</A +></DT +><DT +><A +HREF="x2504.html" +>When the clock is wrong</A +></DT +></DL +></DIV +><BLOCKQUOTE +><P +>“Time is an illusion. Lunchtime double + so.” (Douglas Adams.)</P +></BLOCKQUOTE +><P +> This chapter explains how a Linux system keeps time, + and what you need to do to avoid causing trouble. Usually, + you don't need to do anything about time, but it is good to + understand it.</P +><DIV +CLASS="SECT1" +><H1 +CLASS="SECT1" +><A +NAME="AEN2438" +>Time zones</A +></H1 +><P +> Time measurement is based on mostly regular natural + phenomena, such as alternating light and dark periods caused + by the rotation of the planet. The total time taken by two + successive periods is constant, but the lengths of the light + and dark period vary. One simple constant is noon. </P +><P +> Noon is the time of the day when the Sun is at its + highest position. Since the Earth is round, + + <A +NAME="AEN2442" +HREF="#FTN.AEN2442" +>[1]</A +> + + noon happens at different times in different places. This leads + to the concept of <I +CLASS="GLOSSTERM" +>local time</I +>. Humans + measure time in many units, most of which are tied to natural + phenomena like noon. As long as you stay in the same place, + it doesn't matter that local times differ. </P +><P +> As soon as you need to communicate with distant places, + you'll notice the need for a common time. In modern times, + most of the places in the world communicate with most other + places in the world, so a global standard for measuring time + has been defined. This time is called <I +CLASS="GLOSSTERM" +>universal + time</I +> (UT or UTC, formerly known as Greenwich Mean Time + or GMT, since it used to be local time in Greenwich, England). + When people with different local times need to communicate, + they can express times in universal time, so that there is no + confusion about when things should happen. </P +><P +> Each local time is called a time zone. While geography + would allow all places that have noon at the same time have the + same time zone, politics makes it difficult. For various reasons, + many countries use <I +CLASS="GLOSSTERM" +>daylight savings time</I +>, + that is, they move their clocks to have more natural light + while they work, and then move the clocks back during winter. + Other countries do not do this. Those that do, do not agree when + the clocks should be moved, and they change the rules from year + to year. This makes time zone conversions definitely non-trivial. + </P +><P +> Time zones are best named by the location or by telling + the difference between local and universal time. In the US + and some other countries, the local time zones have a name and + a three letter abbreviation. The abbreviations are not unique, + however, and should not be used unless the country is also named. + It is better to talk about the local time in, say, Helsinki, + than about East European time, since not all countries in Eastern + Europe follow the same rules. </P +><P +> Linux has a time zone package that knows about all + existing time zones, and that can easily be updated when the + rules change. All the system administrator needs to do is to + select the appropriate time zone. Also, each user can set his + own time zone; this is important since many people work with + computers in different countries over the Internet. When the + rules for daylight savings time change in your local time zone, + make sure you'll upgrade at least that part of your Linux system. + Other than setting the system time zone and upgrading the time + zone data files, there is little need to bother about time. + </P +></DIV +></DIV +><H3 +>Notes</H3 +><TABLE +BORDER="0" +CLASS="FOOTNOTES" +WIDTH="100%" +><TR +><TD +ALIGN="LEFT" +VALIGN="TOP" +WIDTH="5%" +><A +NAME="FTN.AEN2442" +HREF="c2432.html#AEN2442" +>[1]</A +></TD +><TD +ALIGN="LEFT" +VALIGN="TOP" +WIDTH="95%" +><P +>According to + recent research.</P +></TD +></TR +></TABLE +><DIV +CLASS="NAVFOOTER" +><HR +ALIGN="LEFT" +WIDTH="100%"><TABLE +WIDTH="100%" +BORDER="0" +CELLPADDING="0" +CELLSPACING="0" +><TR +><TD +WIDTH="33%" +ALIGN="left" +VALIGN="top" +><A +HREF="x2417.html" +>Prev</A +></TD +><TD +WIDTH="34%" +ALIGN="center" +VALIGN="top" +><A +HREF="book1.html" +>Home</A +></TD +><TD +WIDTH="33%" +ALIGN="right" +VALIGN="top" +><A +HREF="x2451.html" +>Next</A +></TD +></TR +><TR +><TD +WIDTH="33%" +ALIGN="left" +VALIGN="top" +>Compressed backups</TD +><TD +WIDTH="34%" +ALIGN="center" +VALIGN="top" +> </TD +><TD +WIDTH="33%" +ALIGN="right" +VALIGN="top" +>The hardware and software clocks</TD +></TR +></TABLE +></DIV +></BODY +></HTML +>
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