Windows sets the BIOS clock to the local time while Linuxes (so elementary OS as well) sets to UTC by default. You can set the hardware clock time standard through the command line. To change this default setting and to set the BIOS clock to local time instead of UTC, you will have to use the timedatectl command. You can check what you have set to use by:
timedatectl | grep local
The hardware clock can be queried and set with the timedatectl command. To change the hardware clock time standard to localtime, use:
timedatectl set-local-rtc 1
If you want to revert to the hardware clock being in UTC, do:
timedatectl set-local-rtc 0
(source: https://askubuntu.com/questions/169376/clock-time-is-off-on-dual-boot)
