Sometimes you need to change the timezone on your Mac to test global location sensitive applications. You can do it via System Preferences but it's quick and easier from the Terminal command line.

Find your current timezone:

sudo systemsetup -gettimezone

Get a list of timezones:

sudo systemsetup -listtimezones

Set your timezone:

sudo systemsetup -settimezone [timezone]

Where [timezone] can be something like Europe/London or GMT