Re: timezones
Clive Menzies <clive@clivemenzies.co.uk> writes:
> On (06/04/05 00:28), Bruno Hertz wrote:
>> michael <linux@networkingnewsletter.org.uk> writes:
>>
>> > Here in the UK we recently went to British Summer Time, putting the
>> > clocks +1hr from GMT. On my dual boot (Debian + WinXP) it now seems I
>> > have a problem. WinXP reports the correct BST time, whereas Debian is an
>> > additional hour in front (ie GMT+2 instead of GMT+1) but I can't work
>> > out why. A quick Internet search didn't throw anything up.
>> >
>> > I'm running:
>> > michael@manchester-campaigns:~$ uname -a;cat /etc/apt/sources.list
>> > Linux manchester-campaigns 2.4.18-bf2.4 #1 Son Apr 14 09:53:28 CEST 2002
>> > i686 GNU/Linux
>> >
>> > deb ftp://ftp.uk.debian.org/debian/ unstable main non-free contrib
>> >
>> > with Real Time Clock Driver v1.10e
>> >
>> > Not sure what other info to add (I thought there were msgs about
>> > hardware clock at boot but not sure how to access these - not in dmesg)
>> >
>> > Ta for any advice!
>>
>> Don't use Windows myself, but I seem to recall that this is a classic
>> dual boot problem with Linux/Windows. More specifically, the default
>> (and reasonable) Linux setup suggests and assumes that your hardware
>> clock is set to UTC (or GMT, if you will, although those are not strictly
>> equivalent), while Windows not only assumes that you use local time, but
>> actually sets your hardware clock this way.
>>
>> What does that mean? During winter, your local time is UTC + 0, so
>> when Windows sets the hardware clock to local time, it is still UTC + 0,
>> and Linux won't be disturbed. When changing to summer time though,
>> Windows sets the hardware clock to UTC + 1. Linux on the other hand
>> still assumes it is UTC + 0, and adds another hour for BST, not on the
>> hardware clock but as system time, which is then hardware clock time + 1.
>> That's the problem :)
>>
>> Afaik, there's nothing to correct this on the Windows side. You can
>> however choose localtime as your default Linux hardware clock time
>> also with hwclock (cf. man hwclock). In this case, the glibc timezone
>> library will _not_ add another hour for BST, and Windows and Linux time
>> should be in sync again. This might have the drawback however that Linux
>> itself won't change any more from GMT to BST or vice versa. At least that's
>> the last I heard of, it might have changed in the meantime. So when that
>> changes occur (last Sunday in March and October), you have to set the
>> hardware clock either yourself on Linux or, hehe, by booting into Windows :)
> Good explanation - I just knew it that they didn't play well together ;)
Well, I thought it wasn't actually that good after reading through it myself.
Too verbose for my taste :) Anyway, I just recently dug through that stuff,
and while not directly related to this particular problem, I can still
recommend the wikipedia pages related to time zones, like
http://en.wikipedia.org/wiki/Coordinated_Universal_Time
http://en.wikipedia.org/wiki/Time_zone
and all the stuff linked from there, i.e. daylight savings and such.
Makes very interesting spare time reading and provides a good general
background. E.g. I learned just there that the US has a 'Mountain
Time Zone', which covers a pretty large area either. I always thought
it was just eastern, central and pacific time, so that was a nice
surprise. Also the difference between UTC and GMT is explained, and lots
of other stuff. All very interesting :)
Regards, Bruno.
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