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Is it the last daylight saving ?

In France, discussions are going well regarding the change of summer and winter time ! Let’s go further in this article..

There is always someone to say that this year is the last time we change the clock :

They want to keep the summer time forever, I heard this on the news, trust me !

How computer science sees it

From the computer science point of view, the daylight savings are a matter of time, and time is the most important thing for a computer. Think about all the servers that performs actions and needs to do it at the right moment.

TZDATA

Time zones are handled by a package called tzdata, the code source can be found here : https://salsa.debian.org/glibc-team/tzdata. Basically this package holds all the rules for all countries in the world in what we call timezone databases.

You can clone it if you want to see what is under the scenes :

c:\dev\src>git clone https://salsa.debian.org/glibc-team/tzdata
Cloning into 'tzdata'...
warning: redirecting to https://salsa.debian.org/glibc-team/tzdata.git/
remote: Enumerating objects: 5983, done.
remote: Counting objects: 100% (1365/1365), done.
remote: Compressing objects: 100% (394/394), done.
remote: Total 5983 (delta 845), reused 1275 (delta 772), pack-reused 4618
Receiving objects: 100% (5983/5983), 3.24 MiB | 2.62 MiB/s, done.
Resolving deltas: 100% (3675/3675), done.

c:\dev\src>cd tzdata

c:\dev\src\tzdata>

In the folder you can find a file for each zone of the world, if you open the one named europe, you can see all the rule for the European continent. The file is a bit huge so you can go directly to line 571 to see the rule that manage the France.

The file explains that the rule for France is the rule of Europe as its belongs to European Union.

# Zone	NAME		STDOFF	RULES	FORMAT	[UNTIL]
Zone	Europe/Paris	0:09:21 -	LMT	1891 Mar 16
			0:09:21	-	PMT	1911 Mar 11 # Paris Mean Time
# Shanks & Pottenger give 1940 Jun 14 0:00; go with Excoffier and Le Corre.
			0:00	France	WE%sT	1940 Jun 14 23:00
# Le Corre says Paris stuck with occupied-France time after the liberation;
# go with Shanks & Pottenger.
			1:00	C-Eur	CE%sT	1944 Aug 25
			0:00	France	WE%sT	1945 Sep 16  3:00
			1:00	France	CE%sT	1977
			1:00	EU	CE%sT

The last line is the one that is active today : 1:00 EU CE%sT.
It basically says that the summer time saves one hour and refers to the rule called EU. The formatted timezone name is CET with an optional character (we will see it below).

Let’s find the EU rule, it is here :

# Rule	NAME	FROM	TO	-	IN	ON	AT	SAVE	LETTER/S
Rule	EU	1977	1980	-	Apr	Sun>=1	 1:00u	1:00	S
Rule	EU	1977	only	-	Sep	lastSun	 1:00u	0	-
Rule	EU	1978	only	-	Oct	 1	 1:00u	0	-
Rule	EU	1979	1995	-	Sep	lastSun	 1:00u	0	-
Rule	EU	1981	max	-	Mar	lastSun	 1:00u	1:00	S
Rule	EU	1996	max	-	Oct	lastSun	 1:00u	0	-

Let’s take a look at the two last lines :

  • Rule EU 1981 max - Mar lastSun 1:00u 1:00 S : starting from the year 1981, the summer time is applied at the last Sunday of March. We can also see that it append the letter S (for summer) that will be used for the formatting.
  • Rule EU 1996 max - Oct lastSun 1:00u 0 : the winter time has been last decide from 1996, it happens the last Sunday of October.

If you are a history lover, you can read the whole part of the file for France and Europe, all the time changes are written. Great work that has been done here !

Rules compilation

When you install tzdata, what it does under the hood is that it is compiling the rules with the zic utility, an other Linux package. This is done at this line of the file tzdata/debian/rules :

# Build the timezone data
	/usr/sbin/zic -d $(TZGEN) -L /dev/null tzdata.zi ;

It will generate each files for each time zones into the directory /usr/share/zoneinfo/. These files seems to be compiled. That’s why we need another tool to get the human readable content, it is called zdump.

Let’s find out what is our timezone on the machine :

[root@test-server ~]# ls -ln /etc/localtime
lrwxrwxrwx. 1 0 0 34 Jul 18  2022 /etc/localtime -> ../usr/share/zoneinfo/Europe/Paris

Let’s now use zump and see what will happen on 2023 :

[root@test-server ~]# zdump -v Europe/Paris | grep 2023
Europe/Paris  Sun Mar 26 00:59:59 2023 UTC = Sun Mar 26 01:59:59 2023 CET isdst=0 gmtoff=3600
Europe/Paris  Sun Mar 26 01:00:00 2023 UTC = Sun Mar 26 03:00:00 2023 CEST isdst=1 gmtoff=7200
Europe/Paris  Sun Oct 29 00:59:59 2023 UTC = Sun Oct 29 02:59:59 2023 CEST isdst=1 gmtoff=7200
Europe/Paris  Sun Oct 29 01:00:00 2023 UTC = Sun Oct 29 02:00:00 2023 CET isdst=0 gmtoff=3600

It is pretty clear :

  • until the 26 March at 0h59 we are 3600 minutes after the global time, so 1 hour, what we call GMT+1.
  • at 1h00 we jump to 7200 minutes after the global time, so 2 hours, what we call GMT+2 aka the Central European Summer Time (CEST). And this until the 29 October at 0h59
  • and then at 1h we come back to GMT+1

Can we see if then have already plan to change it ? Let’s try to see the last lines of the zdump output :

[root@test-server ~]# zdump -v Europe/Paris | tail -5
Europe/Paris  Sun Mar 29 01:00:00 2499 UTC = Sun Mar 29 03:00:00 2499 CEST isdst=1 gmtoff=7200
Europe/Paris  Sun Oct 25 00:59:59 2499 UTC = Sun Oct 25 02:59:59 2499 CEST isdst=1 gmtoff=7200
Europe/Paris  Sun Oct 25 01:00:00 2499 UTC = Sun Oct 25 02:00:00 2499 CET isdst=0 gmtoff=3600
Europe/Paris  9223372036854689407 = NULL
Europe/Paris  9223372036854775807 = NULL

Oh my God ! It is decided until year 2499 ! It is not ready to be be changed ^^

Let’s keep an eye on this

We could create a scrapper that will frequently check for changes in the file that we care about, the europe file. Or maybe try to use a Github Action to do so.

Conclusion

Thanks to computer science, we can answer to the question of time changes with the tzdata package because it holds a very important information for computers : the clock. So basically, if it would change, we should be able to see commits to this package.

To conclude, no, we will not keep the summer time as the only one time !

/posts/is-it-the-last-daylight-saving/ François ALLAIS