[svlug] revised timezone info posted Feb 26

[Brian J. Tarricone]

On Thu, 01 Mar 2007 11:30:48 -0800 Rick Kwan 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That file no longer exists; it has been replaced by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?

The difference between the two isn't in the offset, it's in the time
that the switch occurs.  The TechRepublic example has the time change
wait until 3am, at which time your clock will jump forward to 4am
(which I believe is incorrect).

The one you downloaded for your system will jump to 3am when your clock
hits 2am.  My understanding is that this case is correct.  At least,
according to Wikipedia[1], "the time is adjusted at 2 AM local time."

For what it's worth, the PST8PDT zone info file on my system (Gentoo)
seems to agree with yours (switching at 2am).

	-brian

[1]http://en.wikipedia.org/wiki/Daylight_saving_time_around_the_world#United_States