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<H3 align="left"><img src="../../gx/dennis/qbubble.gif"
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height="50" width="60" alt="(?) " border="0"
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>clock setting</H3>
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<p><strong>From Bryan Henderson
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</strong></p>
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<p></strong></p>
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<!-- sig -->
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<p align="right"><strong>Answered By Bryan Henderson
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</strong></p>
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<blockquote><em><font color="#000066">By this odd chance, the Gang get to be the querents, and we have a real
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guru to answer our clock questions at hand. Thanks Bryan!
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-- Heather</font></em></blockquote>
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<blockQuote>
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<IMG SRC="../../gx/dennis/bbub.gif" ALT="(!)"
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HEIGHT="28" WIDTH="50" BORDER="0"
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>
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As the maintainer of the main Linux hardware clock managing program,
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Hwclock, I found the Answer Gang discussion and survey of daylight
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savings time switches and other hardware clock issues enlightening.
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I'd like to add some important information.
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</blockQuote>
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<P><STRONG>
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<IMG SRC="../../gx/dennis/qbub.gif" ALT="(?)"
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HEIGHT="28" WIDTH="50" BORDER="0"
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> [John Karns]
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Thanx for your contribution! I for one really appreciate it.
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</STRONG></P>
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<P><STRONG>
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<IMG SRC="../../gx/dennis/qbub.gif" ALT="(?)"
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HEIGHT="28" WIDTH="50" BORDER="0"
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> [Ben]
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First thing, Bryan - thank you for the info, as well as for the very useful
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job that you're doing!
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</STRONG></P>
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<P><STRONG>
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<IMG SRC="../../gx/dennis/qbub.gif" ALT="(?)"
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HEIGHT="28" WIDTH="50" BORDER="0"
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> [Mike]
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Yes, Bryan, thanks for taking the time to write that explanation, and
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for offering to debug distribution-caused problems.
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</STRONG></P>
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<blockQuote>
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<IMG SRC="../../gx/dennis/bbub.gif" ALT="(!)"
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HEIGHT="28" WIDTH="50" BORDER="0"
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>
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First, let me state that the _only_ sane reason to keep your hardware
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clock in local time is if you sometimes run Windows on the machine.
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Windows isn't capable of using a hardware clock in any other format.
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Unfortunately, local time is Hwclock's default and the default that
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<A HREF="http://www.redhat.com/">Red Hat</A> and I believe other major distritbutions ship.
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</blockQuote>
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<P><STRONG>
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<IMG SRC="../../gx/dennis/qbub.gif" ALT="(?)"
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HEIGHT="28" WIDTH="50" BORDER="0"
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> [John K]
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How about time zones where daylight savings doesn't apply?
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</STRONG></P>
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<blockQuote>
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<IMG SRC="../../gx/dennis/bbub.gif" ALT="(!)"
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HEIGHT="28" WIDTH="50" BORDER="0"
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>
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Then it's less insane to keep your hardware clock in local time, but still
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not sane.
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</blockQuote>
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<P><STRONG>
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<IMG SRC="../../gx/dennis/qbub.gif" ALT="(?)"
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HEIGHT="28" WIDTH="50" BORDER="0"
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> [Ben]
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I certainly appreciate it; I'm sure that a number of
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our readers do as well. One of your tips in here - the persistence of "UTC"
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- has already let me figure out why my localtime was "backwards" (i.e., 5
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hours earlier instead of later) if I set the hardware clock to UTC. I don't
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use Windows, but I do travel quite a lot, which means I have to keep
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changing time zones; do you have any advice or pertinent info for doing
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this
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</STRONG></P>
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<blockQuote>
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<IMG SRC="../../gx/dennis/bbub.gif" ALT="(!)"
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HEIGHT="28" WIDTH="50" BORDER="0"
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>
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First of all, of course, keep your hardware clock in UTC format.
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Whenever you enter a new timezone, do a quick 'ln' command to link
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<TT>/etc/localtime</TT> to the descriptor for your new timezone.
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</blockQuote>
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<P><STRONG>
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<IMG SRC="../../gx/dennis/qbub.gif" ALT="(?)"
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HEIGHT="28" WIDTH="50" BORDER="0"
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> [Ben]
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Ah, <EM>so</EM>. Actually, I've often thought of writing up a "Mobile Linux"
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article - a sort of a HOWTO for traveling with Linux - and you've just
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cleared up one of the last pieces of the puzzle. <EM>Tres</EM> cool. For those
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folks who need to bounce around as I do, here's something that'll be
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useful:
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</STRONG></P>
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<p align="center">See attached <tt><a href="../misc/tag/chzone1.bash.txt">chzone1.bash.txt</a></tt></p>
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<P><STRONG>
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This script will present you with a menu of choices for the Eastern,
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Central, Mountain, and Pacific timezones. Pick one, and you're set.
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</STRONG></P>
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<blockQuote>
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<IMG SRC="../../gx/dennis/bbub.gif" ALT="(!)"
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HEIGHT="28" WIDTH="50" BORDER="0"
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>
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The <TT>/usr/share/zoneinfo/US</TT> directory may be more appropriate here.
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</blockQuote>
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<P><STRONG>
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<IMG SRC="../../gx/dennis/qbub.gif" ALT="(?)"
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HEIGHT="28" WIDTH="50" BORDER="0"
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> [Ben]
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Odd. The entire "tz*" suite (tzselect, tzconfig, etc.) uses the "America"
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version. <looking at the contents of 'US'> Ah. OK, that seems to make sense
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- at least you'd be setting the timezone by name (
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<IMG SRC="../../gx/dennis/smily.gif" ALT="<grin>"
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height="24" width="20" align="middle"> I'd spent a few
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minutes hopscotching through "tzselect", back and forth, back and forth, to
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figure out which cities it used for which zones.) So, here's an updated
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version of "chzone" - this one actually covers a wider range but keeps the
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choice list down to the actual zones rather than the (possibly confusing)
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list of cities:
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</STRONG></P>
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<p align="center">See attached <tt><a href="../misc/tag/chzone2.bash.txt">chzone2.bash.txt</a></tt></p>
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<blockQuote>
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<IMG SRC="../../gx/dennis/bbub.gif" ALT="(!)"
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HEIGHT="28" WIDTH="50" BORDER="0"
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>
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The C library (GNU libc 2) looks at <TT>/etc/localtime</TT> for the description
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of the local timezone. That can be a symlink to the relevant timezone
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descriptor in <TT>/usr/share/zoneinfo.</TT> (I use US Pacific
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Standard/Daylight time, so I link to <TT>/usr/share/zoneinfo/US/Pacific</TT>).
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If you don't have descriptors for every timezone known to man in
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<TT>/usr/share/zoneinfo</TT> (5 MB of them come with glibc -- having them all
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installed appears to be "normal"), you'll have to install them per
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your distribution. Sometimes they are in <TT>/usr/lib/zoneinfo.</TT>
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</blockQuote>
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<blockQuote>
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Note that changing timezones doesn't cause any time discontinuity.
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You aren't changing the clock, only the language your system uses to
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communicate to humans about what time it is.
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</blockQuote>
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<P><STRONG>
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<IMG SRC="../../gx/dennis/qbub.gif" ALT="(?)"
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HEIGHT="28" WIDTH="50" BORDER="0"
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> [Ben]
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... (hopefully, without screwing up "<TT>/etc/timeadj</TT>") other than setting the
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TZ to the appropriate value? Are there any non-obvious issues with the
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clock that I should be aware of when I do this?
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</STRONG></P>
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<blockQuote>
|
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<IMG SRC="../../gx/dennis/bbub.gif" ALT="(!)"
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HEIGHT="28" WIDTH="50" BORDER="0"
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>
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You change your hardware clock to UTC by adding the --utc option to
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any clock-setting 'hwclock' command. You only have to do it once,
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because your choice gets saved in <TT>/etc/adjtime</TT> and becomes your default
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in the future.
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</blockQuote>
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<blockQuote>
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The major practical drawback to keeping your hardware clock in local
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time is that in most locales, local time jumps an hour twice a year.
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The hardware clock is incapable of implementing that. So you have to
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explicitly reset the hardware clock twice a year. Windows does that
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automatically. In Linux, you can do it with a startup script and/or
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cron job, but I'm not aware of any Linux distribution that does it out
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of the box. If you're running both Linux and Windows, though, I think
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both would make the adjustment!
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</blockQuote>
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<P><STRONG>
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<IMG SRC="../../gx/dennis/qbub.gif" ALT="(?)"
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HEIGHT="28" WIDTH="50" BORDER="0"
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> [John K]
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In my case, the time doesn't change, as I'm near to the equator.
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</STRONG></P>
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<blockQuote>
|
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<IMG SRC="../../gx/dennis/bbub.gif" ALT="(!)"
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HEIGHT="28" WIDTH="50" BORDER="0"
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>
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Actually, the time doesn't change for anybody; only the local time
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representation does.
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<IMG SRC="../../gx/dennis/smily.gif" ALT=":-)"
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height="24" width="20" align="middle">
|
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</blockQuote>
|
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<P><STRONG>
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<IMG SRC="../../gx/dennis/qbub.gif" ALT="(?)"
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HEIGHT="28" WIDTH="50" BORDER="0"
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> [John K]
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OK, but I think you understand what I'm saying - daylight savings time
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doesn't exist here.
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</STRONG></P>
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<P><STRONG>
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<IMG SRC="../../gx/dennis/qbub.gif" ALT="(?)"
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HEIGHT="28" WIDTH="50" BORDER="0"
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> [Mike]
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Where do you live? Indiana?
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</STRONG></P>
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<P><STRONG>
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Why are there not timezone configurations for those locations, and if
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they're not, how hard is it to copy one and modify it to disable the
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DST?
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</STRONG></P>
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<blockQuote>
|
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<IMG SRC="../../gx/dennis/bbub.gif" ALT="(!)"
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HEIGHT="28" WIDTH="50" BORDER="0"
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>
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He said it's near the equator, and he didn't say he can't do timezones
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the normal way (in fact, he probably does). He just pointed out that
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it isn't as advantageous to him as it is to most the world to keep his
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hardware clock in UTC format, because one of the advantages of UTC
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format is that you don't have to reset your hardware clock for DST
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changes.
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</blockQuote>
|
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<P><STRONG>
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<IMG SRC="../../gx/dennis/qbub.gif" ALT="(?)"
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HEIGHT="28" WIDTH="50" BORDER="0"
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> [John K]
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Also,
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the Linux based distributed network I'm setting up, at this time is all
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contained within one time zone. Thus, I haven't felt compelled to leave
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my hw clock set for utc. I did try it once on my personal laptop, (sans
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the --utc option though - I probably used hwclock to set the time, but
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can't remember all the details) but didn't like the fact the timestamps on
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my files (ls) were not in agreement with the time as displayed by the
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'date' command
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</STRONG></P>
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<blockQuote>
|
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<IMG SRC="../../gx/dennis/bbub.gif" ALT="(!)"
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HEIGHT="28" WIDTH="50" BORDER="0"
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>
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The hardware clock format doesn't affect ls and date displays --
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unless there's a bug in the system, of course. I do often see people
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configure their machines for the wrong time zone and then keep the
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hardware clock set to the wrong time to compensate. This causes some
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displays to be correct, but always causes something else to be broken.
|
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</blockQuote>
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<P><STRONG>
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<IMG SRC="../../gx/dennis/qbub.gif" ALT="(?)"
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HEIGHT="28" WIDTH="50" BORDER="0"
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> [John K]
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I've never done that or even considered doing it, as I can see where it
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could really distort parts of the system and create havoc. What I'm
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trying to say here is that, well let me give an example:
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</STRONG></P>
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<P><STRONG><BLOCKQuote>
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<UL>
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<LI> Local time is 1pm
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<LI> I create a file
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<LI> I do an ls, which shows the file with a date-stamp that's skewed e.g., 6 hrs from local time.
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</UL>
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</BLOCKQuote></STRONG></P>
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<P><STRONG>
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Thus I constantly have to do mental arithmetic to figure relate these
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times to my frame of reference, which is local time. It's particularly
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undesirable when those 6 hrs spans midnight, so the date-stamp shows a
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different day.
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</STRONG></P>
|
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<blockQuote>
|
|
<IMG SRC="../../gx/dennis/bbub.gif" ALT="(!)"
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HEIGHT="28" WIDTH="50" BORDER="0"
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>
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And every once in a while you see a program that chooses to display
|
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times in UTC (because it's easy). If you lie to your system about
|
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what time it is, you can trick that program into displaying local
|
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time! But that breaks other programs. If you then lie about what
|
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time zone you're in, those other programs start appearing to work, but
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still other things break. It usually falls apart as soon as you try
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to communicate to the rest of the world, for example email timestamps.
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</blockQuote>
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<P><STRONG>
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<IMG SRC="../../gx/dennis/qbub.gif" ALT="(?)"
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HEIGHT="28" WIDTH="50" BORDER="0"
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> [John K]
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- it tends to make things a little bit confusing. So I
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changed things back to local time.
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</STRONG></P>
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<P><STRONG>
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I do also run Windows but mostly via VMWare on a Linux host. Do you have
|
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any info or input in regards to that scenario?
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</STRONG></P>
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<P><STRONG>
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My main concerns are these:
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</STRONG></P>
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<P><STRONG><BLOCKQuote>
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The distributed net that I'm setting up could eventually span outside of
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the local time zone. When and if that happens, it might make sense to use
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utc.
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</BLOCKQuote></STRONG></P>
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<blockQuote>
|
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<IMG SRC="../../gx/dennis/bbub.gif" ALT="(!)"
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HEIGHT="28" WIDTH="50" BORDER="0"
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>
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If we're still talking about the hardware clock internal format (UTC
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vs local time), I don't think the issues change when you expand into
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multiple time zones. Using local time shouldn't be any worse than
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with one time zone, since Hwclock does all the work of converting
|
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between Unix standard format and hardware clock format.
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</blockQuote>
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<P><STRONG>
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<IMG SRC="../../gx/dennis/qbub.gif" ALT="(?)"
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HEIGHT="28" WIDTH="50" BORDER="0"
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> [John K]
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I have read about the Unix standard format and more or less know what it
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is, but don't really understand the big picture here - how all the parts
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fit together.
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</STRONG></P>
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<blockQuote>
|
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<IMG SRC="../../gx/dennis/bbub.gif" ALT="(!)"
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HEIGHT="28" WIDTH="50" BORDER="0"
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>
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But if you mean set the timezone on all the machines to UTC so that
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displayed times are the same on all systems, that's a separate
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question.
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</blockQuote>
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<P><STRONG>
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<IMG SRC="../../gx/dennis/qbub.gif" ALT="(?)"
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HEIGHT="28" WIDTH="50" BORDER="0"
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> [John K]
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|
But the LANs are a heterogeneous mixture of W9x and Linux clients
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with a Linux server providing application sharing and Internet gateway
|
|
services. I wish to use samba for W9x file sharing and login <TT>/</TT> user
|
|
profile control, as well as run a batch file to sync the clocks on the w9x
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clients to the server clock. In short, I want to have all clocks more or
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less synchronized.
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</STRONG></P>
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<P><STRONG>
|
|
I don't quite follow you here - "displayed times"? .. what about syslogd?
|
|
My concern is mostly with file date-stamps, and system logs. Lets say I'm
|
|
examining a system log of a remote system located in a different time
|
|
zone. I would like to avoid confusion about when specific events may have
|
|
happened in relation to my local time - and this would be my principal
|
|
motivation for using UTC. For example, I will have a "master server"
|
|
which will be doing telephone dialup to remote hosts to exchange mail,
|
|
collect system logs, etc. I would like to have the master server log
|
|
timestamps of the dialup session agree with those of the remote system
|
|
logs, rather than all be skewed one or two hours. Same with file creation
|
|
& modification timestamps. I will likely have a Perl or bash script run
|
|
via cron on remote systems to collect all files of interest having a
|
|
date-stamp falling within a certain time period.
|
|
</STRONG></P>
|
|
<P><STRONG>
|
|
My understanding prior to the test I did at least a year ago when I set my
|
|
hw clock to UTC, was that such date-stamps would be shown (e.g., via ls)
|
|
as local time, but UTC would allow for a standard that would put all
|
|
systems using it on an equal level, and would help to eliminate confusion
|
|
regarding date-stamps on files between different systems. But that didn't
|
|
seem to be the case - it simply added more confusion.
|
|
</STRONG></P>
|
|
<blockQuote>
|
|
<IMG SRC="../../gx/dennis/bbub.gif" ALT="(!)"
|
|
HEIGHT="28" WIDTH="50" BORDER="0"
|
|
>
|
|
The usual way to do that is with an ntp network (run ntpd on all the
|
|
systems. Have a master ntp time server that gets time from some
|
|
higher authority and distributes it down to everyone else). Don't use
|
|
Hwclock at all (I mean it -- if you set the hardware clock manually
|
|
even once, the system won't maintain it automatically after that).
|
|
</blockQuote>
|
|
<P><STRONG>
|
|
<IMG SRC="../../gx/dennis/qbub.gif" ALT="(?)"
|
|
HEIGHT="28" WIDTH="50" BORDER="0"
|
|
> [John K]
|
|
That's what I have been thinking - sync the "master server" clock via NTP
|
|
(ADSL has just recently been introduced here, so now a full time Internet
|
|
connection is possible); then use a system util such as ntpdate or rdate
|
|
(samba logon batch files for the other OS) to sync all other clocks to the
|
|
master. Since my "WAN" will be mostly dialup, using the NTP daemon an all
|
|
servers is not possible or practical.
|
|
</STRONG></P>
|
|
<P><STRONG>
|
|
I still have questions about UTC re: W9x and other flavors under VMWare.
|
|
I guess a little experimentation is in order.
|
|
</STRONG></P>
|
|
<P><STRONG>
|
|
I hope that I haven't rambled too much, and thanx for your input.
|
|
</STRONG></P>
|
|
<P><STRONG>
|
|
Any thoughts you might care to express about this would be greatly
|
|
appreciated.
|
|
</STRONG></P>
|
|
<blockQuote>
|
|
<IMG SRC="../../gx/dennis/bbub.gif" ALT="(!)"
|
|
HEIGHT="28" WIDTH="50" BORDER="0"
|
|
>
|
|
Regardless of in what format you keep your hardware clock, the display
|
|
of the time by 'date', 'ls', etc. is controlled by the time zone
|
|
settings as defined by the C library (e.g. the "localtime" function).
|
|
Remember, the time does not change each Spring and Fall -- only what
|
|
we call the time changes. Properly configured, the C library routines
|
|
generate daylight savings time in Summer and standard time in Winter.
|
|
The underlying clock is oblivious.
|
|
</blockQuote>
|
|
<P><STRONG>
|
|
<IMG SRC="../../gx/dennis/qbub.gif" ALT="(?)"
|
|
HEIGHT="28" WIDTH="50" BORDER="0"
|
|
> [Ben]
|
|
Is the above configuration anything that needs to be done by the local
|
|
admin/user, or does the above mean "properly configured by the
|
|
author/maintainer/etc of the C library"?
|
|
</STRONG></P>
|
|
<blockQuote>
|
|
<IMG SRC="../../gx/dennis/bbub.gif" ALT="(!)"
|
|
HEIGHT="28" WIDTH="50" BORDER="0"
|
|
>
|
|
It's C library installation options, like the <TT>/etc/localtime</TT> setup
|
|
mentioned above.
|
|
</blockQuote>
|
|
<P><STRONG>
|
|
<IMG SRC="../../gx/dennis/qbub.gif" ALT="(?)"
|
|
HEIGHT="28" WIDTH="50" BORDER="0"
|
|
> [Ben]
|
|
Hm. All I can do is hope - now that my hardware clock presumably resembles
|
|
something normal - that the <A HREF="http://www.debian.org/">Debian</A> installation options are right. Heck,
|
|
I'll even go so far as to disable my "spring forward, fall back" cron jobs.
|
|
|
|
<IMG SRC="../../gx/dennis/smily.gif" ALT=":)"
|
|
height="24" width="20" align="middle"> I'm a brave soul, I am.
|
|
</STRONG></P>
|
|
<blockQuote>
|
|
<IMG SRC="../../gx/dennis/bbub.gif" ALT="(!)"
|
|
HEIGHT="28" WIDTH="50" BORDER="0"
|
|
>
|
|
The original querent was having trouble with Ls displaying times in
|
|
UTC instead of local time in Red Hat 6.1. I've dealt with those messy
|
|
time zone problems (There was a totally different way of doing time
|
|
zones before Red Hat 6.0, by the way, and the conversion wasn't
|
|
perfect) many times, and I'd be happy to debug this problem for anyone
|
|
who is having it.
|
|
</blockQuote>
|
|
<P><STRONG>
|
|
<IMG SRC="../../gx/dennis/qbub.gif" ALT="(?)"
|
|
HEIGHT="28" WIDTH="50" BORDER="0"
|
|
> [John K]
|
|
I'm a bit fuzzy on this issue too. What is the expected <TT>/</TT> intended system
|
|
behavior in this regard? If I set my clock to UTC, and specify hwclock's
|
|
--utc parm as you have suggested, then the system should compensate in
|
|
such a way that the ls command would show timestamps reflected as *local
|
|
time* - or UTC?
|
|
</STRONG></P>
|
|
<P><STRONG>
|
|
I suppose that the system always stamps the files in accordance with the
|
|
Unix standard format, and it is up to the various parts of the system (ls,
|
|
tar, and the like) to do conversions in relation to either UTC or local
|
|
time. What I interpret you as saying is that there have been instances
|
|
where these various progs are not in agreement concerning the method with
|
|
which these conversions are done. Am I Correct? I guess it's time for
|
|
another try at setting one of my boxes to UTC to find out what.
|
|
</STRONG></P>
|
|
<P><STRONG>
|
|
I think this was what I was experiencing as well (<A HREF="http://www.suse.com/">SuSE</A> 6.4).
|
|
</STRONG></P>
|
|
<P><STRONG>
|
|
<IMG SRC="../../gx/dennis/qbub.gif" ALT="(?)"
|
|
HEIGHT="28" WIDTH="50" BORDER="0"
|
|
> [Ben]
|
|
A very cool offer indeed - you can't get much better than that if you're
|
|
having problems with the above. I'm not, but - Bryan, my job takes me to
|
|
the Bay area on a fairly regular basis; I'd be more than happy to stand you
|
|
a beer if you're interested, on behalf of all the folks that need and
|
|
appreciate your help.
|
|
</STRONG></P>
|
|
<blockQuote>
|
|
<IMG SRC="../../gx/dennis/bbub.gif" ALT="(!)"
|
|
HEIGHT="28" WIDTH="50" BORDER="0"
|
|
>
|
|
I'd love to.
|
|
</blockQuote>
|
|
<P><STRONG>
|
|
<IMG SRC="../../gx/dennis/qbub.gif" ALT="(?)"
|
|
HEIGHT="28" WIDTH="50" BORDER="0"
|
|
> [Ben]
|
|
Excellent - I'll be up there, let's see <rummaging> the first week of next
|
|
month <waving at Jim and Heather>. See you then!
|
|
</STRONG></P>
|
|
<P><STRONG>
|
|
(Hmm, perhaps the "beerware" concept is outdated. If all of us bought beers
|
|
for all the authors and maintainers, there wouldn't <EM>be</EM> any more authors
|
|
or maintainers - not sober ones, anyway. And where would we be then?
|
|
<IMG SRC="../../gx/dennis/smily.gif" ALT=":)"
|
|
height="24" width="20" align="middle">
|
|
</STRONG></P>
|
|
|
|
<blockquote><em><font color="#000066">Down at the pub, nursing a few sharp ginger beers, or root beers if you like
|
|
them better, until the Guiness wears off and we're safe to drive home.
|
|
<IMG SRC="../../gx/dennis/smily.gif" ALT="8)"
|
|
height="24" width="20" align="middle">
|
|
-- Heather</font></em></blockquote>
|
|
|
|
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