How to check and update planned day light saving (DST) changes (timezone) in Linux

How do I check the planned DST changes for a timezone?

You can get this information from (https://www.timeanddate.com) but you must make sure that your local system is also in sync with the DST changes as showed under (https://www.timeanddate.com)

For example I would like to see the planned DST changes for CET timezone.
From (https://www.timeanddate.com/time/change/germany/berlin) we get the below information

Lets match if with the DST changes available on my system

So we know that my local timeone rpm is capable enough to handle the DST changes.

With RHEL 7 with timedatectl also we can get this information on the planned DST changes

But do we know it will really work?

Let us validate this by manually tweaking our local timezone and date

First change the local timezone to CET, the existing timezone as you see is 'Asia/Kolkata'

Change it to CET

My current date and time

Lets change it to "Sun Mar 25 01:59:59 2018 CET" when we know the DST change should make the system clock shift one hour ahead

On another terminal I have a while loop running to monitor the changing time

If you notice the time changed from 01:59:59 to 03:00:00 because of the planned DST change

Next lets check the DST end changes which as per the timezone is scheduled at 28th oct 2018 when the time shifts back one hour

Using our while loop

So the DST ended with expected timeshift from 02:59:59 to 02:00:00.

What should I do if the timezone (tzdata) rpm does not has planned DST changes?

Many times it can happen that the DST schedule changes without much prior notification, so in such situation you are very much dependent on NTP but what if you don't have NTP server?
In that case you have to make sure you have the latest timezone (tzdata) rpm which has the new changes for the specific timezone.

For Red Hat you can get the list of changes done for individual tzdata rpm under below page
https://access.redhat.com/articles/1187353

In case your vendor has not yet released a tzadata rpm file and you need a new fix then you can always download it from the main source of tz database

For latest available tzdata
https://www.iana.org/time-zones

If you want to access older tzdata archive
ftp://ftp.iana.org/tz/

For the sake of this article I will give an example from a recent scenario
In the year 2016 Turkey government announced not to have DST changes anymore so the old tzdata rpm was not aware of this change hence if not updated it will continue to shift the time as per the old planned DST changes

My existing tzdata rpm

which is currently unaware that now for Turkey timezone there should be no more DST changes after the year 2016

As you see if I check the planned DST changes for the year 2017, it still shows me that the DST will start on '26th March' from 'EET' to 'EEST' and will end on '29th Oct' from 'EEST' to 'EET' again.

To fix this we need updated tzdata rpm with the necessary changes, this was updated in 2016g tzadata rpm so I downloaded the same from IANA database (ftp://ftp.iana.org/tz/)

and copied the same to my setup

next extract the needed timezone file here

In the NEWS file you should get the information regarding the Turkey time changes

Lets extract the needed timezone file and place it on our system

This will create a directory zoneinfo and will extract all the timezone files under europe
Here we will have 'Istanbul' timezone which is same as Turkey, overwrite the existing Istanbul timezone with the new one

If you observe I only modified Istanbul timezone but my 3 files are updated

So now lets see if this timezone has the updated information about the new time changes from Turkey government.

First lets check for the year 2017

As expected there are no planned DST changes in the year 2017 as Turkey government ended the DST in 2016 itself

For the year 2016 if you will compare the output from our last old tzdata rpm

The DST will end on Sep 7 and the timezone will change from EEST to '+03' instead of 'EET'

I hope the article was useful.