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

Brian McKee brian.mckee at
Sat Mar 28 00:55:32 UTC 2009

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?


More information about the ubuntu-users mailing list