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[20091024]
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Fun around daylight saving time (DST) and Unix timezone handling in general
It's that time of the year again when I and my friends never
know when daylight saving time is switched from summer-
to winter time. Do you know? And if you've heared in the
news today... do you know when the next change is?
There's an easy way to find out, using the
zdump(8)
utility that comes with about every Unix machine (at least
with NetBSD and Mac OS X - is there anything else relevant? :),
and the timezone information stored in the /usr/share/zoneinfo
directory.
To find out when DST is switched on/off, run the following command:
% zdump -v /etc/localtime
...
/etc/localtime Sun Mar 29 00:59:59 2009 UTC = Sun Mar 29 01:59:59 2009 CET isdst=0
/etc/localtime Sun Mar 29 01:00:00 2009 UTC = Sun Mar 29 03:00:00 2009 CEST isdst=1
/etc/localtime Sun Oct 25 00:59:59 2009 UTC = Sun Oct 25 02:59:59 2009 CEST isdst=1
/etc/localtime Sun Oct 25 01:00:00 2009 UTC = Sun Oct 25 02:00:00 2009 CET isdst=0
/etc/localtime Sun Mar 28 00:59:59 2010 UTC = Sun Mar 28 01:59:59 2010 CET isdst=0
/etc/localtime Sun Mar 28 01:00:00 2010 UTC = Sun Mar 28 03:00:00 2010 CEST isdst=1
/etc/localtime Sun Oct 31 00:59:59 2010 UTC = Sun Oct 31 02:59:59 2010 CEST isdst=1
/etc/localtime Sun Oct 31 01:00:00 2010 UTC = Sun Oct 31 02:00:00 2010 CET isdst=0
/etc/localtime Sun Mar 27 00:59:59 2011 UTC = Sun Mar 27 01:59:59 2011 CET isdst=0
/etc/localtime Sun Mar 27 01:00:00 2011 UTC = Sun Mar 27 03:00:00 2011 CEST isdst=1
/etc/localtime Sun Oct 30 00:59:59 2011 UTC = Sun Oct 30 02:59:59 2011 CEST isdst=1
/etc/localtime Sun Oct 30 01:00:00 2011 UTC = Sun Oct 30 02:00:00 2011 CET isdst=0
...
The data is given relative to Universal Time Coordinated, which
you can determine on a NetBSD system by running "date -u":
% date
Sat 24 Oct 14:30:24 CEST 2009
% date -u
Sat 24 Oct 12:30:26 UTC 2009
And if you wonder what timezone you're actually in, you probably knokw that /etc/localtime is a
symbolic link to a file that fits to your exact
timezone, with all related information:
% ls -la /etc/localtime
lrwxr-xr-x 1 root wheel 33 May 26 2007 /etc/localtime -> /usr/share/zoneinfo/Europe/Berlin
If you always wanted to know what timezones there are, have a look
at /usr/share/zoneinfo:
% ls /usr/share/zoneinfo/
Africa Chile GB-Eire Israel NZ-CHAT UCT
America Cuba GMT Jamaica Navajo US
Antarctica EET GMT+0 Japan PRC UTC
Arctic EST GMT-0 Kwajalein PST8PDT Universal
Asia EST5EDT GMT0 Libya Pacific W-SU
Atlantic Egypt Greenwich MET Poland WET
Australia Eire HST MST Portugal Zulu
Brazil Etc Hongkong MST7MDT ROC iso3166.tab
CET Europe Iceland Mexico ROK posixrules
CST6CDT Factory Indian Mideast Singapore zone.tab
Canada GB Iran NZ Turkey
% ls /usr/share/zoneinfo/Europe/
Amsterdam Chisinau Kiev Moscow Sarajevo Vatican
Andorra Copenhagen Lisbon Nicosia Simferopol Vienna
Athens Dublin Ljubljana Oslo Skopje Vilnius
Belfast Gibraltar London Paris Sofia Volgograd
Belgrade Guernsey Luxembourg Podgorica Stockholm Warsaw
Berlin Helsinki Madrid Prague Tallinn Zagreb
Bratislava Isle_of_Man Malta Riga Tirane Zaporozhye
Brussels Istanbul Mariehamn Rome Tiraspol Zurich
Bucharest Jersey Minsk Samara Uzhgorod
Budapest Kaliningrad Monaco San_Marino Vaduz
So now that we all that, here's a last cutie: suppose you want
to log into a machine on the other end of the world, and still
run an application that should use your local timezone, not
that that the machine is in. Changing /etc/localtime is not
an option, but you can do it on a per-process base by setting
the "TZ" environment variable:
% date
Sat Oct 24 14:35:06 CEST 2009
% ssh remote.example.org date
Sat Oct 24 12:35:45 UTC 2009
% ssh remote.example.org env TZ=Europe/Berlin date
Sat Oct 24 14:35:58 CEST 2009
Enjoy!
[Tags: date, dst, localtime, utc, zdump]
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