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Dual boot system Windows XP Pro and Ubuntu 12.04.

I have the bios set for the correct time and Ubuntu set for US Eastern time. Ubuntu will boot up and the time will be off by -4 hours. If I correct the time in Ubuntu then when I boot up in Windows XP the time will be off by + 4 hours.

This is a fresh install of 12.04. I did not have this problem before reinstalling Ubuntu.

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4 Answers 4


Operating systems store and retrieve the time in the hardware clock located on your motherboard so that it can keep track of the time even when the system does not have power. Most operating systems (Linux/Unix/Mac) store the time on the hardware clock as UTC by default, though some systems (notably Microsoft Windows) store the time on the hardware clock as the 'local' time. This causes problems in a dual boot system if both systems view the hardware clock differently.

The advantage of having the hardware clock as UTC is that you don't need to change the hardware clock when moving between timezones or when Daylight Savings Time (DST) begins or ends as UTC does not have DST or timezone offsets.

Changing Linux to use local time is easier and more reliable than changing Windows to use UTC, so dual-boot Linux/Windows systems tend to use local time.

Since Intrepid (8.10), UTC=yes is default.

Make Windows use UTC:

Note: This method was not initially supported on Windows Vista and Server 2008, but came back with Vista SP2, Windows 7 and Server 2008 R2.

To make MS Windows calculate the time from the hardware clock as UTC.

Create a file named WindowsTimeFixUTC.reg with the following contents and then double click on it to merge the contents with the registry:

Windows Registry Editor Version 5.00


Make Linux use 'Local' time:

To tell your Ubuntu system that the hardware clock is set to 'local' time:

  1. edit /etc/default/rcS
  2. add or change the following section
#Set UTC=yes if your hardware clock is set to UTC (GMT)
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But I already have UTC=no in my /etc/default/rcS. I have UTC=no and have the problem. –  Green Aug 2 '13 at 5:34
Note how awkward it is to fix it in Windows and how comparatively straightforward it is to fix it in Linux... :) –  Lagerbaer Oct 29 '13 at 16:15
Oh lord. I can't believe they were dumb enough to use local time on a hardware clock. Why would you ever, ever do that? –  iono Dec 9 '13 at 7:22
@twome because since there wasn't much in the way of networking in the early days of MS-DOS, and it wouldn't have been common to move a PC so far that it moved timezones. (And yeah, even considering that history, it was still not the best move). –  Jon Hanna Apr 22 '14 at 13:44
@Laqerbaer, changing a registry setting is as trivial for a Windows user as changing a config file is for a Linux user. So spare us the petty sniping. –  Alan B May 22 '14 at 18:58

Your time zone is Eastern, which is currently EDT (Eastern Daylight Time). EDT is UTC minus four hours, the same as the offset you're experiencing.

When this happens on a dual-boot system, it's usually because one operating system thinks the hardware clock tracks local time, while the other operating system thinks the hardware clock tracks UTC.

In your case, your hardware clock is probably set to the local time, and:

  • Windows is set to use local time, which (given your time settings) is correct.
  • Ubuntu is set to use UTC, which (given your time settings) is incorrect.

If you were to just reset the time in the Ubuntu system, either manually or automatically, it would then be wrong in Windows, so that's not a good solution.

Instead, the best solution is probably to reconfigure Ubuntu to treat the hardware clock time as local time (then you can leave your Windows configuration, and your hardware clock time, alone).

To do this, edit /etc/default/rcS as root and make sure it has UTC=no:

  1. Press Alt+F2.

  2. Type gksu gedit /etc/default/rcS and press Enter.

  3. You'll probably see this:

    # assume that the BIOS clock is set to UTC time (recommended)
    • If you do, change UTC=yes to UTC=no. (Or you might want to add a comment too, by changing it to something like UTC=no # changed to accommodate Windows system. Everything on a line after a # character is a comment, and is there just to make your settings more human-readable.)
    • If you don't, look for any UTC= line. If it's uncommented (i.e., doesn't have a # at the beginning), change it accordingly. If it is commented, uncomment it by removing the leading # and make sure it says UTC=no.
  4. Save the file and quit the text editor.

  5. Reboot to apply your changes and check that the time is working properly now on both operating systems.

Source: https://help.ubuntu.com/community/UbuntuTime#Make_Linux_use_.27Local.27_time

(But that is very general; I have written this answer to apply specifically to your situation, which is a somewhat common problem.)

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The /etc/rcS fix did not work on my Win7 / Ubuntu 14.04x64 installation, nor did any fixes through the clock GUI. Because the root of the problem is BIOS time VS UTC time, and Windows is a pain in the butt when it comes to using UTC, I just picked a point on the map that uses UTC+0 (no change between BIOS and UTC time).

Try changing your location to Reykjavik (the capital of Iceland). Voila! Your Ubuntu time will change to match your BIOS time.

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Haha! Awesome. :D I really don't care my location. This was two seconds fix. Thanks a lot. –  bludger Mar 10 at 9:05


Click the clock in the toolbar and click Date & Time Settings.

On the Clock tab. check the box for Time in auto-detected location and the clock should correct immediately."

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protected by Community Apr 18 '14 at 15:49

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