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I have a dual boot machine. Ubuntu and Windows Vista. After rebooting from Ubuntu to Vista, i see that the time has changed in Vista and vice versa. Why does this happen?? The time zones on both the OS are correct.

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(Tweaked tags as this isn't Vista (nor Ubuntu) specific. I used to see this e.g. with Windows 2000 and Fedora. The cause is always the same; see e.g. Al's answer.) –  Jonik Sep 15 '09 at 12:16

3 Answers 3

up vote 4 down vote accepted

Look at this answer:

Serverfault

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It's probably because Ubuntu is configured to assume that the system clock is GMT/UTC (or whatever you want to call it) and it adjusts the time zone for the location of the user that is logging in. Windows changes the actual system clock to match the time zone. You can reconfigure Ubuntu to assume that the system clock is local time, but I can't remember how to do it at the moment (google will tell you!).

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This is due to a bit of confusion with how the two different operating systems store and retrieve the current time. Some operating systems (or BIOS?), by default, will 'store' and 'retrieve' the time in the BIOS using the UTC timezone and then convert it to the user's desired timezone, whereas others may 'store' and 'retrieve' it in the user's timezone.

This problem is common (or most visible) by Mac users using bootcamp.See this post from the Mac Observer forum:

In your Windows registry, you need to update the key:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\TimeZoneInformation\RealTimeIsUniversal

Once you get down there, the RealTimeIsUniversal setting should be a DValue of 1. That registry key tells Windows that the time of the internal clock kept in the NVRAM, where your settings are, is using Coordinated Universal Time (UTC), sometimes (incorrectly) referred to as Greenwich Mean Time (GMT).

NOTE: I know this works for XP but I think it works for Vista, too. I have not used Win7 so don’t even ask!

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It's the OS, not BIOS. The same thing happens when dual booting Windows and Mac OS X, so than rules out the BIOS as being responsible. –  alex Sep 15 '09 at 12:15
    
OS X machines still have a BIOS, but it is more or less emulated. And when I say BIOS (in the post), I am really referring to the CMOS, where the time is 'kept', or wherever that happens. –  David Pearce Sep 15 '09 at 12:39
    
You're right, the EFI is similar to the BIOS. –  alex Sep 15 '09 at 13:14
    
No, I'm just saying that as a part of Boot Camp, Apple installs a BIOS compatibility later (of sorts) –  David Pearce Sep 15 '09 at 13:23

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