For some strange reason, additional timezones cannot be added to the clock using the date-time indicator in Gnome Classic (Ubuntu 12.04). I used Unity before I switched to Gnome Classic, and it was possible to add more timezones. Even in Gnome Classic I can see the other timezones in the menu of the date-time indicator which I added when I was still using Unity. When I go to System Settings > Date and Time, there is no option for adding other timezones. How can I set additional timezones in Gnome Classic?
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The unity-indicator as you may have gathered does not have the same capabilities as Unity. The developers have patched the gnome-libraries to give this integration. However - by design - they have left the Gnome-Shell & Gnome-Classic desktops as vanilla as possible. This is by expression wish by the gnome-developers.
Thus you will need a slightly more inventive solution - this answer will give you one such suggestion.
use the clock applet
Temporarily add the clock applet to the panel.
Find your location by clicking edit in the applet
Next look carefully at the locations:
The value you are interested in is timezone city
In this example:
America/Port_of_Spain Piarco
Europe/Rome Verona
Next fire up dconf-editor
dconf-tools 
Look at the locations field as shown and add your new location(s) - each entry is in the format ,'timezone city' i.e. separate entries with commas
You then will automatically update your date-time indicator. You can finish off by deleting the clock applet.
answered Jun 23 '12 at 20:53
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2Thanks a lot. As medigeek says in the other answer, the list of timezone strings is at en.wikipedia.org/wiki/List_of_tz_database_time_zones so you may not need the clock applet. Also, just an obvious note for anyone else doing this: after editing the Locations list, do tick "Show locations" as well! – ShreevatsaR Jan 21 '13 at 7:06
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This seems to be the only way to set multiple time locations in Ubuntu 15.04. The official GUI stops working for me after I add the first location (no drop down, a red circle with a white bar sign if I try to type it in). If anyone else is experiencing that, let me know, and I will file a bug on launchpad. – Greg Kramida Jul 12 '15 at 15:52
If the "Choose locations" is missing from the Date and Time control but it does work, you should file a bug about it:
ubuntu-bug gnome-session-fallback
You can actually edit it manually through dconf-editor:
sudo apt-get install dconf-tools
dconf-editor
Select com > canonical > indicator > datetime
Look at the locations value.
['UTC UTC', 'Asia/Nicosia Nicosia']
Purely guessing, I think the first (e.g. Asia/Nicosia) is the time zone name and the second (e.g. Nicosia) is the name that appears in the indicator menu (tray icon).
You can find the list of time zones here: https://en.wikipedia.org/wiki/List_of_tz_database_time_zones
answered Jun 23 '12 at 20:42
