Any idea why this happends and how to fix this?
I once had the same problem, and I know why this happens when you have a dual-boot setup with Windows and Linux. It's because, by default, Windows and Linux treat your internal RTC differently. Windows expects the RTC to be set to your local time, while Linux expects it to be set to UTC.
The thing is, I forgot how I fixed it exactly...

I do remember using some commands in the Linux terminal to tell Linux to use local time.
I have used duelboot before, but this has never happened with me. Normally I would use a secondary device to test different flavor of distro, but this time I wondered why not duel boot.
I have installed Arch btw! (am I being too dramatic?)

Anyway, thank you for the "keywords", RTC UTC and those, it helped, and I have found the fix..