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I installed Linux Mint first on my Acer Aspire 4930 and then dual booted with Windows 7. I see the correct time on Linux Mint but on booting into Windows the time is shifted back by a few hours, even after re-setting the time on rebooting it shows the wrong time again. Why is this happening?

On Windows, the system RTC clock is traditionally kept in local time. In Unix and Linux, it's traditionally kept in UTC, and /etc/localtime is used to indicate the current timezone so that the displayed time is correct.

These two worldviews collide in dual-boot configurations, because there's only one RTC. Usually you tell Linux to assume that the RTC is local time (because Windows can't be told). This varies from one Linux to another, but since Mint is like Ubuntu you can probably set UTC=no in /etc/default/rcS. Then you can set the clock to local time in Windows and Linux will leave it alone after that.

The most generic way of doing it is with hwclock --systohc --localtime which is persistent across reboots and works on most systems.

Mint 18.2 KDE seems to be immune to the other fixes (including additional ones I found in a search). The files mentioned in aecolley's and error404's answers either no longer exist or seem to no longer be where the setting is referenced, at least in the current KDE version. The command in Julie Pelletier's answer worked for 18.2 Cinnamon but not 18.2 KDE.

After trying half a dozen suggested solutions, the following fixed it for 18.2 KDE. With the correct time displayed, run the following terminal command:

timedatectl set-local-rtc 1 --adjust-system-clock

Source: Tips and Tricks for Linux Mint after Installation [Mint 18 - Cinnamon Edition]

This should also work for Cinnamon as an alternate solution, and I assume other DE's as well.

WARNING

This method is buggy in pre-7 versions of Windows, but it resolves the issue on the Windows side of the equation, rather than on the Linux end: you don't have to set Linux to use localtime.

Taken from the most glorious and esteemed Arch Linux Wiki:

One reason users often set the RTC in localtime is to dual boot with
Windows (which uses localtime).

However, Windows [can handle the RTC being UTC] with a simple registry fix. It is recommended to configure Windows to use UTC, rather than Linux to use localtime.

Using regedit, add a DWORD value with hexadecimal value 1 to the registry:

HKEY_LOCAL_MACHINESYSTEMCurrentControlSetControlTimeZoneInformationRealTimeIsUniversal

You can do this from an Administrator Command Prompt running:

reg add "HKEY_LOCAL_MACHINESystemCurrentControlSetControlTimeZoneInformation" /v RealTimeIsUniversal /d 1 /t REG_DWORD /f

Alternatively, create a *.reg file (on the desktop) with the following content
and double-click it to import it into registry:

Windows Registry Editor Version 5.00



[HKEY_LOCAL_MACHINESYSTEMCurrentControlSetControlTimeZoneInformation]

     "RealTimeIsUniversal"=dword:00000001

If the above appears to have no effect, and a 64-bit variant of Windows is being used, using a
QWORD value instead of a DWORD value may resolve the issue.


Should
Windows ask to update the clock due to DST changes, let it. It will
leave the clock in UTC as expected, only correcting the displayed
time.


The hardware clock and system clock time may need to be updated
after setting this value. If you are having issues with the offset of
the time, try reinstalling tzdata and then setting your time zone
again:

# timedatectl set-timezone America/Los_Angeles

Amend (as root) /etc/timeadj variable from UTC to LOCAL.

This sorted the problem without having to fiddle with regedit on the Windows side.

Run one of these two command which match your local time

timedatectl set-local-rtc 1 --adjust-system-clock

'or'

timedatectl set-local-rtc 0 --adjust-system-clock