You will need to install
sudo apt-get install dconf-editor
2. Set your Locations
Once installed, go to
com.canonical.indicator.datetime and look for
locations variable and add your desired locations, separated with commas, and between ' '.
They should look something like these:
['America/Los_Angeles Los Angeles', 'America/New_York New York', 'Europe/London London', 'Europe/Paris Paris', 'Asia/Tokyo Tokyo']
If you don't know timezone names, you can go to:
TZ database- Wikipedia
3. Enable Locations
show-locations checkbox. To disable, you can simply uncheck this option.