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RHEL handling of DST change

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Most server hardware clocks are use UTC. UTC stands for the Universal Time, Coordinated, also known as Greenwich Mean Time (GMT). Other time zones are determined by adding or subtracting from the UTC time. Server typically displays local time, which now is subject of DST correction twice a year. 

Wikipedia defines DST as follows:

Daylight saving time (DST), also known as summer time in British English, is the convention of advancing clocks so that evenings have more daylight and mornings have less. Typically clocks are adjusted forward one hour in late winter or early spring and are adjusted backward in autumn. 

DST patch is only required in few countries such as USA.  Please see this wikipedia article.

Linux will change to and from DST when the HWCLOCK setting is set to `-u', i.e. when the hardware clock is set to UTC (which is closely related to GMT), regardless of whether Linux was running at the time DST is entered or left.

When the HWCLOCK setting is set to `--localtime', Linux will not adjust the time, operating under the assumption that your system may be a dual boot system at that time and that the other OS takes care of the DST switch. If that was not the case, the DST change needs to be made manually.

Note:

EST is defined as being GMT -5 all year round. US/Eastern, on the other hand, means GMT-5 or GMT-4 depending on whether Daylight Savings Time (DST) is in effect or not.

The tzdata package contains data files with rules for various timezones around the world. When this package is updated, it will update multiple timezone changes for all previous timezone fixes.

To retrieve the most recent version of tzdata package, run the command:

yum update tzdata

After updating the timezone package, the system must run its timezone configuration package to update the obsolete /etc/localtime file:
   1.
Run the command as the superuser root:
     # system-config-date

   2.
Choose the timezone tab, ensure the correct country and locale are selected for this system.

   3. Click OK.

====================

Uncheck "Use local time source" and on the time zone tab, check "system clock uses UTC".


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Old News ;-)

[Oct 26, 2013] Daylight Savings Time check on Redhat

See also http://www.novell.com/communities/files/Aus-Time-Zone-08.txt

The command I'm using to check that DST will change at the right time reads out of the tzdata file:

zdump -v Australia/Sydney | grep 2008

It returns output like so:

Australia/Sydney Sat Apr 5 15:59:59 2008 UTC = Sun Apr 6 02:59:59 2008 EST isdst=1 gmtoff=39600
Australia/Sydney Sat Apr 5 16:00:00 2008 UTC = Sun Apr 6 02:00:00 2008 EST isdst=0 gmtoff=36000
Australia/Sydney Sat Oct 4 15:59:59 2008 UTC = Sun Oct 5 01:59:59 2008 EST isdst=0 gmtoff=36000

Australia/Sydney Sat Oct 4 16:00:00 2008 UTC = Sun Oct 5 03:00:00 2008 EST isdst=1 gmtoff=39600

Good for checking that your clocks are in order!

Howto Redhat enterprise Linux setup DST Daylight Saving Time

Wikipedia defines DST as follows:
Daylight saving time (DST), also known as summer time in British English, is the convention of advancing clocks so that evenings have more daylight and mornings have less. Typically clocks are adjusted forward one hour in late winter or early spring and are adjusted backward in autumn.

DST patch is only required in few countries such as USA. Please see this wikipedia article. It display usage and a short history of daylight saving time by location in alphabetic order.

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Managing Accurate Date and Time

1. Why do We Need a Precise Clock?
1.1. On What Factors Does the Hardware-Clock Depend?
2. Computer Global Date and Time Concept
3. What are Time Zones?
3.1. Daylight Savings Time
3.2. Time Zones Examples
3.3. Time Zone Mechanism on Linux
4. The Correct Settings for Your Linux Box
4.1. Setting Time Zone
4.2. Setting the Hardware Clock
5. Accurate Global Time Synchronization
5.1. NTP: The Network Time Protocol
5.2. Building a Simple Time Synchronization Architecture
5.3. NTP Configurations
5.4. Watching Your Box Synchronizing
5.5. Configure to Automatically Run NTP at Boot
6. Precise Time with the chrony Program
6.1. How chrony Differs from the ntp Suite?
6.2. How to Use chrony?
A. About this Document

Keeping Time

14.1. The concept of localtime
14.2. The hardware and software clocks
14.3. Showing and setting time
14.4. When the clock is wrong
14.5. NTP - Network Time Protocol
14.6. Basic NTP configuration
14.7. NTP Toolkit
14.8. Some known NTP servers
14.9. NTP Links



Etc

The Last but not Least Technology is dominated by two types of people: those who understand what they do not manage and those who manage what they do not understand ~Archibald Putt. Ph.D


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