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In my previous post, I mentioned I was hit by a complete data loss on a ntfs partition shared by Windows 7 and Ubuntu 12.04, for some unclear reason.

A strange thing happened at the same time is that the time shown by both OSes were changed to be 4 hours in advance at the same time when data loss happened.

  1. In Ubuntu, I wanted to change the time back to normal (EDT). To my surprise, I found the settings are:

    Location: New York 
    Set the time: Automatically from the internet

    The location is correct for EDT, and I have always been having internet connection. Just don't understand why the time is 4 hours in advance?

  2. What might possibly be the reason that the time change happened to both OSes instead of just Windows, when data loss happened during rebooting from Windows into Ubuntu?


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The time change likely happened because one OS thinks the hardware clock should be programmed with UTC time (likely Ubuntu) and the other OS thinks the hardware clock should be programmed with local time (likely Windows).

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Thanks! So UTC time and local time are different, but the times under both OSes are both 4 hours in advance? – SteveO Sep 13 '12 at 23:33
Most likely, the flag for UTC or local time on the hardware clock was changed causing the OS to write the clock in one format and now both OSes to expect to read the clock in another format. – David Schwartz Sep 14 '12 at 0:16
Yep, one OS thinks the hardware clock is timezone A and the other thinks it's timezone B. I never could get Windows and Linux to agree on this, especially with DST (which they would adjust for in addition to the hardware adjusting for, leading to some bizarre time changes). It's the sort of thing you can figure out for about 5 minutes, then it doesn't make sense again. – Daniel R Hicks Sep 14 '12 at 1:27

The underlying system clock is actually in UTC. New York is UTC-5 standard time or UTC-4 daylight saving time. Ubuntu had adjusted the underlying clock so that the your computer's onboard clock is in fact in UTC, causing Windows to indicate the wrong time, but it appears that Ubuntu no longer thinks the system clock is in UTC and incorrectly began to treat it as local time.

For consistency with Windows, configure both operating systems to treat the system as either local time or UTC. See this Ask Ubuntu question for more details.

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