How to change my system time?

My system time is one hour behind of real world time. I tried to change it on Windows 8.1 by changing settings of date and time but after reboot it changes to one hour behind again. After that I tried to change time zone so I can at least have correct time but system again changed it to one hour behind. Also I tried to change it in BIOS and that haven't worked too. What do I have to do?
