cvs commit: hints/PREVIOUS_FORMAT time.txt

tushar at tushar at
Sat Oct 4 09:04:04 PDT 2003

tushar      03/10/04 10:04:04

  Added:       .        time.txt
               OLD      time.txt
  Removed:     PREVIOUS_FORMAT time.txt
  Added Hint: time
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  Index: STATUS
  RCS file: /home/cvsroot/hints/MAINTAINER/STATUS,v
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      * sylpheed: Could not contact author.
      * tacops: No response from author.
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      * uclibc-bootfloppy: Conversion in progress.
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  1.1                  hints/OLD/time.txt
  Index: time.txt
  TITLE:		Time on LFS
  AUTHOR:		Marc Heerdink <marc at>
  	This is an in-depth explanation of timezones, UTC, the TZ environment variable and similar topics.
  version 2.1 (04/10/2002)
  1 Introduction
  2 Local vs. UTC
  3 Determining your timezone
  4 Using the timezone
    4.1 Creating symlinks
    4.2 Changing bootscripts
        4.2.1 SYSV Init
        4.2.2 BSD Init
    4.3 TZ environment variable
  5 Time synchronization
    5.1 Picking an NTP server
    5.2 Synchronize it
    5.3 Maintenance
    Earth is divided in 24 time zones. The local time in a zone depends on how
  many zones that zone is away from Greenwich, Great-Britain. The difference in
  time is relative to the 0-zone over Greenwich. For example, in Holland (where I
  live) we're in the +1-zone (that means, it's one hour later here than in Great
  Britain). This compensates for the world turning.
    Your Linux system is multinational. It can not only talk different languages,
  but it's aware of timezones too. This hint tells you how you can set it up.
    The second chapter of this hint is probably the most important one, the rest
  of this hint depends on the choice you make in chapter 2. Good luck... :)
  Send comments, suggestions, love-letters and improvements to:
  Marc Heerdink <marc at>
    The first and most important question you'll have to answer is whether you
  want to store the time in your machine in either UTC or local time format. UTC
  is Greenwich time, local time is the time that is displayed on a clock hanging
  on a wall near you (tm). Each format has it's own advantages and disadvantages,
  but both of them are discussed in this hint.
    Traditionally, all POSIX machines (i.e. Solaris boxes, BSD Machines but Linux
  boxes too) have their system time in UTC (Coördinated Universal Time) format.
  Stupider OSes require their users to configure their machines for local time,
  and these OSes are mainly the Microsoft ones. Fortunately, Linux can handle
  both the normal UTC machines and the machines suffering from Microsoft diseases
  that have their system time in local format. I still recommend UTC, that's the
  way Linux was originally set up. Usage of local time is only a workaround.
    At this point, you'll have to decide what it's gonna be: local or UTC time.
  Some guidelines: If you're running Windows and Linux together on 1 box, I
  recommend you use local time, if you have Windows but you hardly use it or if
  you don't have Windows at all, it's a good idea to store your time in UTC
  format. Both ways are described here.
    Knowing what timezone you're living in, is important for the rest of this
  hint. But it's not enough to know how many zones you're away from Greenwich,
  since daylight saving is also influenced by this choice. LFS comes with an
  easy program to determine your timezone in only a few questions (usually 2 or
  3). Run it now:
    When this program quits, the last line it prints is your timezone. Here, it
  prints "Europe/Amsterdam" (without the quotes) because I live in the
  Netherlands. Remember this value; write it down or put it somewhere in a text
  file. This variable will be referenced to as _TIMEZONE_ in the rest of this
  hint to simplify explanations.
    Now that you know what timezone you're living in, we can put this knowledge
  into practice. This chapter deals with the various ways a timezone needs to be
  set up. Each subchapter describes what we're going to do and tells you how. I
  assume you're using a virgin LFS installation. Otherwise, you'll have to do
  some improvisation.
    These are symlinks that have to be present, this is not different for either
  people with local time or people with UTC time. Why these symlinks have to be
  created, will be explained in chapter 4.2. For now, just enter the
  /usr/share/zoneinfo directory and create a symlink from localtime to your
  timezone with the following command:
   cd /usr/share/zoneinfo &&
   ln -sf _TIMEZONE_ localtime
    Chapter 3 describes this _TIMEZONE_ thingie. Read it if you didn't already do
  so, it contains importand information. Next, we need another sylink in /etc:
   cd /etc &&
   ln -sf ../usr/share/zoneinfo/localtime
    The symlinks are now set up, changing bootscripts is next.
    At boot time, the system clock that is maintained by the Linux kernel, has
  to be synchronized to the hardware clock. The hardware clock is the clock that
  ticks in your BIOS and keeps the time even if the system is powered time. The
  reason to do so, is that the hardware clock is not really precise. The Linux
  kernel uses a far more precise way of time keeping, that I'm not going to
  describe in detail here. To keep the Linux system time when the system goes
  down, the system time has to be written to the hardware clock when the system
  is going to reboot or power down.
    Writing to the hardware clock and reading from it is performed by a program
  named hwclock(8). This program is part of a normal LFS system, so you don't
  need to download it. We'll modify some of the bootscripts to make this program
  run at power up and power down.
    This chapter describes the changes that have to be made to either the SYSV
  bootscrips that are described in the book, or to the BSD style bootscripts
  that are described in the BSD Init hint. If you don't know what type you have,
  you can assume you're using the SYSV type bootscripts.
  4.2.1 SYSV INIT
    In the book, a script named setclock is created in /etc/init.d/setclock is
  created. This script performs only half of the job, because it only sets the
  system time from the hardware clock. We will modify this script and create
  some additional symlinks to make this script save the system time to the
  hardware clock if needed. But first, you'll have to check if the
  /etc/sysconfig/clock file is correct. This file defines a variable UTC. What
  value this variable gets, is pretty self-explanatory after reading chapter 2.
  Summarized: UTC=1 for UTC hardware clocks, UTC=0 for localtime clocks. Next:
  we create a new /etc/init.d/setclock:
   rm -f /etc/init.d/setclock
   cat >/etc/init.d/setclock <<EOF
  # Begin /etc/init.d/setclock
  # Include the functions declared in the /etc/init.d/functions file
  # and include the variables from the /etc/sysconfig/clock file
  source /etc/init.d/functions
  source /etc/sysconfig/clock
  case "$1" in
  		case "$UTC" in
  				/sbin/hwclock --hctosys --utc
  				/sbin/hwclock --hctosys --localtime
  				echo "Invalid value for UTC in /etc/sysconfig/clock: $UTC"
  				echo "Valid values for UTC are 1 and 0."
  				exit 1
  		case "$UTC" in
  				/sbin/hwclock --systohc --utc
  				/sbin/hwclock --systohc --localtime
  				echo "Invalid value for UTC in /etc/sysconfig/clock: $UTC"
  				echo "Valid values for UTC are 1 and 0."
  				exit 1
  		echo "Usage: $0 {start|stop}"
  		exit 1
  # End /etc/init.d/setclock
   chmod 755 /etc/init.d/setclock
    Next are the symlinks. The symlink to run the setclock script is already
  present in /etc/init.d/rcS.d, so the only symlinks we have to create are the
  ones to run setclock when the system shuts down:
   cd /etc/rc0.d &&
   ln -sf ../init.d/setclock S700setclock &&
   cd /etc/rc6.d &&
   ln -sf ../init.d/setclock S700setclock
    At this point, the bootscripts are correctly set up and the only thing that's
  left to configure is the TZ environment variable in chapter 4.3.
  4.2.2 BSD INIT
    The scripts are hwclock aware out of the box. The only thing you'll have to
  change, are the parameters for hwclock in the /etc/rc.d/rc.sysinit and
  /etc/rc.d/rc.0 scripts. Open these scripts in a text editor and search for the
  lines that call hwclock. These lines should look like
  for UTC time:
  (rc.sysinit) /sbin/hwclock --hctosys --utc
  (rc.0)       /sbin/hwclock --systohc --utc
  for local time:
  (rc.sysinit) /sbin/hwclock --hctosys --localtime
  (rc.0)       /sbin/hwclock --systohc --localtime
    And that's it!
    This variable is used by hwclock when it's run from a shell, and some
  programs that heavily depend on timezones. This variable is used system-wide,
  so it's a good idea to have it in the system-wide environment that is set up in
  /etc/profile. Add these lines to your /etc/profile:
   export TZ
    Time synchronization means "making your system display the correct time".
  You'll learn how to synchronize your time with one of the NTP (Network Time
  Protocol) servers out there and how to keep this time synchronized. In this
  chapter, we'll use a handy utility named "getdate" to do the time
  synchronization. Get it from:
    Now extract it in /usr/src:
   cd /usr/src &&
   tar xzvf /path/to/getdate_rfc868-1.2.tar.gz
    And compile it:
   cd getdate_rfc868 &&
   make &&
   make install &&
   make installman
    This is everything we need for time synchronization.
    This is the most difficult part of time synchronization. You need an NTP
  server that is preferably in your timezone, and that is working. Pick one from
  the list @ Test if it works
   getdate your.ntp.server
    If you don't get a positive output, pick the next NTP server from the list and
  try it again, otherwise, adjust your system time manually to a value as close as
  possible to this ntp server. For example, if you get this:  (-40)  Sun Mar 4 13:33:33 2001
    Adjust your time with this command:
   date -s 13:33:40
    It's not really important yet what time exactly you set it to, but we need a
  value that is close enough for getdate, since it won't work with a greater
  difference between NTP and local time than 180 seconds.
    Type this to synchronize your system time with the NTP server:
   getdate -adjust 5 180 your.ntp.server
    This is it! The correct time will be written to your hardware clock next time
  your system shuts down (well... if you read chapter 4) so you'll have a quite
  correct time for a few days. "A few days?!" Yes, because your hardware clock
  isn't really precise. To make sure you always have the correct time, you'll have
  to periodically synchronize it again. How this can be done will be explained in
  chapter 5.3.
    You can keep your system clock in shape if you regularly synchronize your
  local clock with an NTP server. It can be a good idea to do this every time you
  connect to the internet, or (if you have a continuous connection) with a cron
  job. If you don't like this automation, you can of course do it manually from
  time to time. This chapter contains some examples for the given methods.
    First method is "synchronize when the internet link goes up." In this
  example, the standard PPP daemon is used, because it's often used for serial
  links. Open the file /etc/ppp/ip-up in a text editor. This file gets auto-
  matically executed when the ppp link goes up so it's safe to synchronize from
  here. Add this line to the end:
   /usr/bin/getdate -adjust 5 180 your.ntp.server
    The second method is "synchronize every x hours using a cron job." There are
  too many cron daemons out there to give an example that works for every daemon.
  Normally, the man page for the crontab editor you use will give you enough
  information to be able to do this yourself. If you have made such a cronjob for
  one particular cron daemon, please let me know and I'll add it here.
    The third and last "do it yourself"-method is pretty straightforward. Run the
  familiar getdate -adjust line every once in a while...

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