Re: Time quandry when dual boot with WinXP Pro
On Fri 21 Oct 2016 at 14:28:26 (+0100), Darac Marjal wrote:
> On Fri, Oct 21, 2016 at 08:09:38AM -0500, Richard Owlett wrote:
> >I'm creating a preseed.cfg file for installing Debian 8.6 in a
> >dual-boot enviroment with some version of MS Windows. There are
> >two distinct use cases:
> > [snipped]
>
> If you're not using NTP to provide your time, then, at boot, the
> operating system will query the "hardware clock" to see what time it
> is. As the hardware clock can drift, operating systems will also
> set the hardware clock to match the "system clock" at shutdown time
> (the system clock is the time that the operating system is using).
>
> The problem comes that there is no way for the hardware clock
> (HWClock) to report what timezone it was set to. There are basically
> two options here:
>
> 1. The HWClock is set to "Local Time". In this case, the operating
> system sees "12:23" as being "12:23 GMT" or "12:23 CST" or whatever
> and attempts no translation of the clock.
>
> 2. The HWClock is set to "Universal Time" (which is a
> non-geographical datum time zone, with no daylight changes). In this
> case, the operating system sees "12:23" as ONLY being "12:23 UTC"
> and translates that into the local time zone by adding or
> subtracting hours appropriately.
>
> If you only have one operating system on your machine, either of
> these methods is fine. If you have TWO (or more) operating systems,
> though, then they SHOULD agree on what the time in the HWClock
> represents. If Windows thinks it's local time, but Debian thinks
> it's UTC, then you'll see five-hour shifts each time you boot.
>
> You've already found the setting in Debian, I see, so you probably
> want to check it tallies with what Windows is using. For that, look
> at the Registry Key HKLM\SYSTEM\CurrentControlSet\Control\TimeZoneInformation\RealTimeIsUniversal.
> If this is 1, then the HWClock is using UTC; if it is 0, then the
> HWClock is set to local time.
So what happens with the scenario:
. No NTP connection,
. Hardware clock running on Local Time,
. Two or more OSes that know these two facts.
. Neither OS was running when Civil Time changed to/from DST,
. After the time changed, one OS has run, updating the Local Time to match Civil Time,
. Another OS is just being booted up.
What Local Time will eventually be displayed by this system?
Cheers,
David.
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