Windows/Ubuntu Dual Boot-Setting time in one changes time in the other

Dirk Freitag unreal.linux at
Sat Mar 28 01:00:23 UTC 2009

Brian McKee wrote:
> On Fri, Mar 27, 2009 at 8:48 PM, Dirk Freitag <unreal.linux at> wrote:
>> Brian McKee wrote:
>>> On Fri, Mar 27, 2009 at 6:51 PM, Andrew Johnson <ajj9 at> wrote:
>>>> I am dual booting Ubuntu 8.10 and Windows 7 beta.  The time in Windows
>>>> is always 4 hours ahead of Ubuntu.  Whenever I change the time in one it
>>>> also changes it in the other.  The BIOS clock shows the correct time.
>>>> I've tried changing the time zone and setting time via the Internet, but
>>>> the times are still always 4 hours apart.
>>> Your in Eastern under Daylight savings then :-)
>>> Yep - Windows expects the hardware clock to be in local time, and Unix
>>> usually wants UTC.
>>> I believe the fix is to set in /etc/default/rcS the line UTC=yes to
>>> UTC=no and reboot into Windows.  Make sure the clock is right there,
>>> then boot back into Ubuntu.
>>> Brian
>> Brian,
>> I am confused as to how the two OSes can affect one another like that.
>> If you are not booted into Ubuntu, it shouldn't matter what the time is
>> set to in Windows and visa versa.  He was saying that if he changes the
>> time in Windows, then the time in Ubuntu changes as well.  This
>> absolutely makes no sense to me.
> The BIOS is the base clock and keeps time when the OS is off.
> He boots into Windows, and the time is out.  He either fixes it by
> hand or via time server, and Windows helpfully 'fixes' the time on the
> hardware clock on the motherboard in BIOS.  Then he reboots into
> Ubuntu, and it picks up the time from the motherboard and sets the OS
> to that time.  It's out by 4 hours because Linux expects the hardware
> clock to be UTC rather than EDT.  In Ubuntu he either fixes the time
> manually or by ntp time server, then when he shuts down Linux
> helpfully 'fixes' the hardware clock.  And around we go...
> Note you don't get this when you run one or the other virtualized -
> just when you dual boot.
> Does that explain it better?
> Brian

Yes it does.  I was not aware that the OSes changed the time in the 
BIOS.  You learn something new everyday.  Thank you for the clarification.

Dirk Freitag
Linux Registered User Number 487244          []

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