最近遇到一个有关timezone的问题：有些application需要环境变量TZ，可是我们刚刚装好的RHEL没有设置TZ，而我又没有root权限去改，查了半天资料，发现所有对timezone的改动都需要重启机器才能生效，没有办法即时生效……Anyway，还是把查到的资料罗列在下面，以备将来用到。

Linux Clock Configuration
Howto: Linux server change or setup the timezone

原文：http://www.linuxselfhelp.com/quick/clock.html

Linux Clock Configuration

Author: Edward Buck
Version: .2
Last edited: December 19, 2002

Summary

This guide will help you setup the correct date and time on your Red Hat 7.x Linux system including setting up ntpd for online time synchronization.

Instructions

There are two clocks to configure in Linux, the hardware clock and the system clock. The hardware clock determines the system clock on system boot. While the system is running, changes to one of these doesn't affect the other.

Note: it's best to set the hardware clock and have the system clock be set upon a reboot. Changing the system clock by using the date program on a running system could cause date discontinuities and consequently problems. If you will be using ntpd, you probably don't need to set either of these clocks (unless the current time is more than 1000s off the real time). Just setup ntpd and let ntpd adjust the time (it will do it in small steps to keep system timestamps reliable).

If convenient, use Red Hat's dateconfig tool. Using the dateconfig tool will update both the system clock and the hardware clock. The dateconfig tool also allows you to setup ntpd, which will keep the system clock in sync with a remote server.

If using the dateconfig tool is not an option, follow the remaining steps to configure manually. For example, if you are using a non graphical terminal (dateconfig requires X windows), you must configure manually.
You can use timeconfig to configure the timezone and UTC settings. Timeconfig will update /etc/sysconfig/clock and /etc/localtime.
Set the time zone manually if not already set.
Linux uses the file /etc/localtime to determine the time zone. This file should be either a copy of the appropriate timezone file from the directory /usr/share/zoneinfo or a symbolic link. If your time zone is incorrect, create a symbolic link to the appropriate timezone file.
```
# ln -s /usr/share/zoneinfo/America/Los_Angeles /etc/localtime
```
Set whether hwclock uses local time or UTC
Edit the file /etc/sysconfig/clock and change "UTC=" to true or false. If you have a dual-boot system with Windows, using UTC may cause problems for Windows.
Set the system clock
```
# date MMDDhhmmCCYY.ss
```
where MM is month, DD is day, hh is hour, mm is minutes, CCYY is year and ss is seconds. Time should be in 24-hour notation.

To only set the time:
```
# date -s hh:mm:ss
```
Set the hardware clock
To set the hardware clock to the current system clock:
```
# setclock
```
this method looks at /etc/sysconfig/clock to determine whether the hardware clock is set to UTC

Another method:
```
# hwclock --systohc# hwclock --systohc --utc
```
use the second option if you use UTC.

Set hwclock manually:
```
# hwclock --set --date="9/22/96 16:45:05"
```
Everytime you use the hwclock --set command, it will create or edit the file /etc/adjtime to determine the systematic drift. Once you have some history, you can use the --adjust option to adjust the hardware clock appropriately. Run as a cron job if you want the clock to adjust automatically on a regular schedule. Don't use the --adjust function when using ntpd since ntpd will turn the "11 minute mode" on, which is best left alone. See the hwclock manpage for more info.
Setup ntpd for automatic synchronization with a remote server.
Run Red Hat's setup utility to make ntpd start on boot up and edit /etc/ntp.conf

Set server and fudge options:
```
server time.nist.govfudge  time.nist.gov stratum 10
```
Enable multicastclient:
```
multicastclient     # listen on default 224.0.1.1
```
Edit /etc/sysconfig/ntpd if necessary. The default should be fine.

Start the ntpd daemon:
```
# service ntpd start
```

原文：http://www.cyberciti.biz/faq/howto-linux-unix-change-setup-timezone-tz-variable/

Howto: Linux server change or setup the timezone

by VIVEK GITE on AUGUST 27, 2006 · 32 COMMENTS

Q. My timezone is pointing to wrong timezone. How do I setup or change the timezone under Linux operating systems?

A. Unix time, or POSIX time, is a system for describing points in time: it is the number of seconds elapsed since midnight UTC on the morning of January 1, 1970, not counting leap seconds.

The definition for time zones can be written in short form as UTC±n (or GMT±n), where n is the offset in hours.

Change Linux timezone

Select the method as per your Linux distribution:

If you are using Fedora / RHEL / Cent OS Linux

Type the redhat-config-date command at the command line to start the time and date properties tool.
# redhat-config-date
OR type setup and select time zone configuration (good for remote ssh text based Linux server sessiob)
# setup

Select timezone configuration

Fig.01: Redhat / CentOS Server Setting Up Timezone

Now, just follow on screen instructions to change timezone.

Set timezone using /etc/localtime configuration file [any Linux distro]

Often /etc/localtime is a symlink to the file localtime or to the correct time zone file in the system time zone directory.

Generic procedure to change timezone

Change directory to /etc
# cd /etc

Create a symlink to file localtime:
# ln -sf /usr/share/zoneinfo/EST localtime
OR some distro use /usr/share/zoneinfo/dirname/zonefile format (Red hat and friends)
# ln -sf /usr/share/zoneinfo/EST localtime
OR if you want to set up it to IST (Asia/Calcutta):
# ln -sf /usr/share/zoneinfo/Asia/Calcutta localtime
Please mote that in above example you need to use directory structure i.e. if you want to set the timezone to Calcutta (India) which is located in the Asia directory you will then have to setup using as above.

Use date command to verify that your timezone is changed:
$ date
Output:

Tue Aug 27 14:46:08 EST 2006

Use of environment variable

You can use TZ environment variable to display date and time according to your timezone:
$ export TZ=America/Los_Angeles $ date
Sample Output:

Thu Aug 27 11:10:08 PST 2006

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