[svlug] revised timezone info posted Feb 26

[Joel Seidman]

--- Rick Kwan <kwanrj03 at comcast.net> wrote:

> I just updated my machine at work for the new Daylight Savings Time, which
> starts March 11, and ran into a little surprise along the way.  Many web 
> pages
> say to grab the new timezone file via:
>     wget ftp://elsie.nci.nih.gov/pub/tzdata2007b.tar.gz
> That file no longer exists; it has been replaced by 'tzdata2007c.tar.gz'.
> Internally, it looks like 'northamerica' was updated on Feb. 26, 2007.
> 
> There is an article at TechRepublic:
>     http://articles.techrepublic.com.com/5100-10877_11-6163042.html
> which shows the following example for Mountain Time after update.
> --------
> /etc/localtime  Sun Mar 11 08:59:59 2007 UTC = Sun Mar 11 02:59:59 2007 
> MST isdst=0 gmtoff=-25200
> /etc/localtime  Sun Mar 11 09:00:00 2007 UTC = Sun Mar 11 04:00:00 2007 
> MDT isdst=1 gmtoff=-21600
> /etc/localtime  Sun Nov  4 07:59:59 2007 UTC = Sun Nov  4 02:59:59 2007 
> MDT isdst=1 gmtoff=-21600
> /etc/localtime  Sun Nov  4 08:00:00 2007 UTC = Sun Nov  4 02:00:00 2007 
> MST isdst=0 gmtoff=-25200
> --------
> 
> But when I execute the following command for Mountain Time
>     /usr/sbin/zdump -v /usr/share/zoneinfo/MST7MDT | grep 2007
> I get something different:
> --------
> /usr/share/zoneinfo/MST7MDT  Sun Mar 11 08:59:59 2007 UTC = Sun Mar 11 
> 01:59:59 2007 MST isdst=0 gmtoff=-25200
> /usr/share/zoneinfo/MST7MDT  Sun Mar 11 09:00:00 2007 UTC = Sun Mar 11 
> 03:00:00 2007 MDT isdst=1 gmtoff=-21600
> /usr/share/zoneinfo/MST7MDT  Sun Nov  4 07:59:59 2007 UTC = Sun Nov  4 
> 01:59:59 2007 MDT isdst=1 gmtoff=-21600
> /usr/share/zoneinfo/MST7MDT  Sun Nov  4 08:00:00 2007 UTC = Sun Nov  4 
> 01:00:00 2007 MST isdst=0 gmtoff=-25200
> --------
> Note the assignments of UTC to MST.  The two sets of results
> differ by an hour, but "gmtoff" is unchanged.
> 
> I assume that Linux basically ignores local time, and runs all its time
> services off of UTC.  If that's not true, then things might be really
> interesting. :-)
> 
> Anyone have better insight into this?

It looks to this uninformed observer that the second set is legally correct, and the first set
would cause a one-hour error during one-hour intervals on March 11 and  Nov. 4 (at times I plan to
be asleep).

Regardless of how Linux keeps time internally, it may be used for tasks that relate to local time,
from just displaying it, to taking some local-time-dependent actions. If you're like me, it won't
matter, but for some it does, and they will definitely want to have the correct time-zone data.

BTW, I have a question. My understanding is Linux can run two ways, one where it manages the PC
clock as UTC, and one where it expects the PC clock to reflect the local time (as when it dual
boots with Windows). Can someone tell me where this is controlled? Is it distro dependent? Or am I
confused?

-- Joel


 
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