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Re: DST, will Fedora change the time on the fly?

On Sun, 2009-03-08 at 15:10 -0400, David wrote:
> On 3/8/2009 12:34 PM, Tom Horsley wrote:
> > On Sun, 08 Mar 2009 12:02:34 -0400
> > David wrote:
> >>> As long as you don't multi-boot Windows, of course :-). I've never found
> >>> any combination of settings in Windows to make it not "fix" the system
> >>> clock (even when I tried to run Windows in timezone GMT, it still
> >>> decided it had to fiddle the system clock the first time I booted
> >>> after a DST change).  
> >> That is set on the timezone configuration page of the clock settings. A
> >> check-box.
> > I know. I've checked that checkbox and unchecked that checkbox, and waited
> > 6 months for a new DST change, and it has screwed up every time :-).
> > I have the ultimate solution now - I no longer multi-boot windows :-).
> That works too.  :-)
> Seriously though I have to use Windows from time to time for work
> related stuff and that check-box, left unchecked, has worked for me.

You still need to be careful.  Here's how I understand things working:

      * If Fedora is set to local time and running at the DST change, it
        will change the time, and save the new time to the hardware
        clock on shutdown/reboot.
      * If Fedora is set to local time and not running at the DST
        change, it will read the hardware clock on boot, which will be
        wrong if you didn't change it manually or Windows didn't adjust
      * If Fedora is set to UTC, it will keep the hardware clock in UTC
        and do the right thing with respect to DST whether it was
        running or not.
      * So if you set your hardware clock to local time and Windows is
        running at DST or is the first OS you boot after DST with the
        machine off, the clock will be updated (or not) according to the
        Windows setting, then Linux will not adjust when you boot it.
        So depending on what OS is running at the change, what OS is
        booted first after the change, and what setting the hardware
        clock has, you can end up anywhere from 0 to 2 hours off forward
        or backward after the change.

I boot Windows rarely, so I like the following strategy:

I have the hardware clock (and Linux) set to UTC.  I have the Windows
timezone set to GMT with the DST conversion turned off.  (You need to do
this for *every* windows user!)  Then Linux always does the right thing,
and Windows always leaves the clock alone.  And file timestamps are
always correct in both systems, in case you cross-mount filesystems.

The only disadvantage is that when Windows is running, it displays GMT
with no DST adjustment.  So you need to adjust the clock mentally or
wear a watch (or carry a phone or PDA).

                Matthew Saltzman

Clemson University Math Sciences
mjs AT clemson DOT edu

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