Region & time zone settings in Linux Mint 15 Cinnamon
For some weird reason, in Linux Mint 15 Cinnamon, Region & city selection boxes have been removed from Cinnamon Settings > Calender. You can set your region during installation, but if you make a mistake during installation or travel between timezones with a laptop then indeed there seems to be no easy way to configure this. This issue has been reported already as an issue.
In the meantime, you can change it from the command line.
Display available time zone options: