[TOC]
Mostly for RHEL5 systems. Should work on any Linux distro.
Preliminary notes
-----------------
The config for `ntpd` is `/etc/ntp.conf`. These lines are of interest:
server 0.centos.pool.ntp.org
server 1.centos.pool.ntp.org
server 2.centos.pool.ntp.org
They must be changed to whatever you have in mind for NTP servers.
**Important**: You *must* also include these servers in the
`/etc/ntp/step-tickers` file! This file will be found initially empty.
If you don't, you'll see something like this when restarting the NTP
daemon:
[root@localhost ntp]# /sbin/service ntpd start
ntpd: Synchronizing with time server: [FAILED]
Starting ntpd: [ OK ]
The NTP port is **123** and must be open in your iptables rules to allow
synchronization.
The University of Iowa NTP servers
----------------------------------
As mentioned earlier, these need to be in the `step-tickers` file.
ntp1.uiowa.edu
ntp2.uiowa.edu
ntp3.uiowa.edu
How Time Zones work in Linux
----------------------------
**Important**: The NTP servers do *not* provide timezone-specific data!
All they provide is UTC time. This data is then adjusted by `ntpd` in
conjunction with `/etc/localtime`.
The `/etc/localtime` file has a list of the zone offsets (going till
2038; the 32-bit problem) and, importantly, the dates on which the clock
is moved forward and backward (for the Daylight Savings crap.) To see
the contents of this file, you have to use `zdump`:
zdump -v /etc/localtime | grep 2010
This will show how the clock is adjusted w.r.t. timezone and DST:
/etc/localtime Sun Mar 14 07:59:59 2010 UTC = Sun Mar 14 01:59:59 2010 CST isdst=0 gmtoff=-21600
/etc/localtime Sun Mar 14 08:00:00 2010 UTC = Sun Mar 14 03:00:00 2010 CDT isdst=1 gmtoff=-18000
/etc/localtime Sun Nov 7 06:59:59 2010 UTC = Sun Nov 7 01:59:59 2010 CDT isdst=1 gmtoff=-18000
/etc/localtime Sun Nov 7 07:00:00 2010 UTC = Sun Nov 7 01:00:00 2010 CST isdst=0 gmtoff=-21600
Comprehensive timezone resources are located in `/usr/share/zoneinfo`.
It's the directory utilized by, for example, Anaconda to set your
timezone at install. Here's a sample tree view of the contents of this
folder.
America
|-- Anguilla
|-- Antigua
|-- Araguaina
|-- Argentina
| |-- Buenos_Aires
| |-- Catamarca
| |-- ComodRivadavia
| |-- Cordoba
| |-- Jujuy
| |-- La_Rioja
| |-- Mendoza
| |-- Rio_Gallegos
| |-- Salta
| |-- San_Juan
| |-- San_Luis
| |-- Tucuman
| -- Ushuaia
|-- Aruba
|-- Asuncion
|-- Atikokan
.
.
.
To manually adjust your timezone, you just need to symlink the correct
resource file to `/etc/localtime`.
For US zones, the naming scheme is slightly weird. Here's a table of
files for four US timezones. They are at the root of
`/usr/share/zoneinfo`
| Time Zone | File in `/usr/share/zoneinfo` |
|-----------|-------------------------------|
| Eastern | `EST5EDT` |
| Central | `CST6CDT` |
| Mountain | `MST7MDT` |
| Pacific | `PST8PDT` |
Again, use `zdump` to view/verify these files.
Adjusting Time Zones
--------------------
### The simple way
You just have to symlink the correct time resource file to
`/etc/localtime`. For instance, if I'm in the America/Chicago timezone:
mv /etc/localtime /etc/localtime.backup
ln -s /usr/share/zoneinfo/America/Chicago /etc/localtime
service ntpd restart
### tzselect
If you don't want to bother with pesky symlinking to update
`/etc/localtime`, you can always use `tzselect`. It's a small,
interactive command-line utility which will generate a string like this
that you can add to `~/.bash_profile`
TZ='Africa/Porto-Novo'; export TZ
Fixing Daylight Savings Issues
------------------------------
A `zdump` of the `/etc/localtime` file will yield what the local system
does at certain dates.
zdump -v /etc/localtime
Look at the output. If you see the correct dates for Daylight Savings,
you're good. For example,
/etc/localtime Sun Mar 14 07:59:59 2010 UTC = Sun Mar 14 01:59:59 2010 CST isdst=0 gmtoff=-21600
/etc/localtime Sun Mar 14 08:00:00 2010 UTC = Sun Mar 14 03:00:00 2010 CDT isdst=1 gmtoff=-18000
/etc/localtime Sun Nov 7 06:59:59 2010 UTC = Sun Nov 7 01:59:59 2010 CDT isdst=1 gmtoff=-18000
/etc/localtime Sun Nov 7 07:00:00 2010 UTC = Sun Nov 7 01:00:00 2010 CST isdst=0 gmtoff=-21600
Which seems correct, since the time jumped ahead an hour on March 14.
But here's some worrisome output:
/etc/localtime Sun Apr 4 07:59:59 2010 UTC = Sun Apr 4 01:59:59 2010 CST isdst=0 gmtoff=-21600
/etc/localtime Sun Apr 4 08:00:00 2010 UTC = Sun Apr 4 03:00:00 2010 CDT isdst=1 gmtoff=-18000
/etc/localtime Sun Oct 31 06:59:59 2010 UTC = Sun Oct 31 01:59:59 2010 CDT isdst=1 gmtoff=-18000
/etc/localtime Sun Oct 31 07:00:00 2010 UTC = Sun Oct 31 01:00:00 2010 CST isdst=0 gmtoff=-21600
In this case, you'll need to update the timezone files in
`/usr/share/zoneinfo`.
**Note:** The timezone files are all compiled with [`zic`](http://www.gsp.com/cgi-bin/man.cgi?section=8&topic=zic)
### The easy way - Use YUM
One way to do it is to update the `tzdata` RPM.
yum update tzdata
This should fetch and install the correct files.
### The hard way - Compile with Zic
Download the latest data. This is the file as of 2010:
wget ftp://elsie.nci.nih.gov/pub/tzdata2010f.tar.gz
They don't extract to a directory, so make sure you create one! Now
compile zone files using `zic` with this syntax:
zic -d
Let's try it for North America. The file to compile is called
`northamerica`. Let's compile the results to `tempdir`
zic -d tempdir northamerica
Now verify that the newly compiled files have the correct DST offsets:
[root@localhost tempdir]# zdump -v tempdir/CST6CDT | grep 2010
CST6CDT Sun Mar 14 07:59:59 2010 UTC = Sun Mar 14 01:59:59 2010 CST isdst=0 gmtoff=-21600
CST6CDT Sun Mar 14 08:00:00 2010 UTC = Sun Mar 14 03:00:00 2010 CDT isdst=1 gmtoff=-18000
CST6CDT Sun Nov 7 06:59:59 2010 UTC = Sun Nov 7 01:59:59 2010 CDT isdst=1 gmtoff=-18000
CST6CDT Sun Nov 7 07:00:00 2010 UTC = Sun Nov 7 01:00:00 2010 CST isdst=0 gmtoff=-21600
Woohoo! Now you can copy these over to `/usr/share/zoneinfo`:
[root@localhost tempdir]# cp -r * /usr/share/zoneinfo/
Now remove the old `/etc/localtime` and symlink the newly compiled zone
file:
[root@localhost tempdir]# rm -f /etc/localtime
[root@localhost tempdir]# ln -s /usr/share/zoneinfo/CST6CDT /etc/localtime
Restart `ntpd` and you're good to go!
Resources
---------
1. [Update Linux and FreeBSD systems for new Daylight Saving Time
setting](http://articles.techrepublic.com.com/5100-10878_11-6163042.html)
2. [Date, Time and Time Zones for Red Hat
Linux](http://www.vanemery.com/Linux/RH-Linux-Time.html) - Excellent
overview of NTP and date and time utilities.