With Bootcamp, it’s easy to install Windows on a Mac, but there are problems. Even with the latest Bootcamp 5 Apple released few weeks ago, they still remain unfixed. For instance, you will notice that when you reboot your Mac switching between Mac and Windows your Windows’s clock is always off (unless you live at GTM+0 area). In fact, it’s always off by your local GMT offsets. If you live on the west cost of North America, you will notice the hour is off by 7 to 8 hours, depending on where you are. This is because in Mac and most Linux they use GMT time where as in Windows they use something else, so every time when your Mac reboots into Windows it tries to convert the value from GMT.
How to Fix Windows To Display Correct Clock
Go launch Windows regedit.exe
Right click add a new DWORD(32-bit) value key
Name the key to RealTimeIsUniversal, give it a value 1
When you done as the above, on your next restart, your Windows will getting the correct time instead off by your GMT offsets.