[ntp:questions] Automatic time synchronization of local hw clock.

Martin Burnicki martin.burnicki at meinberg.de
Tue Apr 22 08:32:27 UTC 2014

Rob schrieb:
> Martin Burnicki <martin.burnicki at meinberg.de> wrote:
>> Imagine what happens if you shut down Windows *before* DST starts and
>> reboot *after* DST has started? Your system time will be off by 1 hour
>> because standard time has been written to the RTC at shutdown, but DST
>> is assumed to be read from the RTC at boot time, while the RTC in fact
>> was still running at local standard time.
> No, because Windows recognizes this condition and adjusts accordingly.

Maybe this depends on the Windows version.

> What really fouls things up, though, is when you boot Linux inbetween
> and have the RTC set to localtime there as well.

Of course, since Linux can't know what Windows has done.

> I remember having read somewhere that there is a registry setting
> in Windows to have the RTC in UTC as well, but it is not documented
> in the usual Windows reference manuals.

That is what I meant in my previous post:
>> The obvious approach to achieve this would be to convince Windows
>> to write UTC to the RTC chip at shutdown, and expect UTC from the
>> RTC at startup. There is some registry key which should instruct
>> windows to do so.
>> However, the last time this was discussed here someone mentioned
>> that the registry key isn't honored by the Windows OS as expected.
>> I'm not sure if Windows used local time anyway, or if Windows
>> didn't write to the RTC at all when the registry key was present.
>> In any case it didn't work as supposed to. Google may help you to
>> find this discussion.

Here is some general information on this topic:

Markus Kuhn: IBM PC Real Time Clock should run in UT

> Normally you can avoid these issues when you have an NTP server
> that is reachable at boot time, and have NTP setup in such a way that
> it allows large timesteps on the initial setting of time.

Of course you can in most cases find a workaround which gets things fixed.

Martin Burnicki

Meinberg Funkuhren
Bad Pyrmont

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