I am looking for the same. Haven't found the solution, but found a couple of things that are close;
a) In the 'add to panel' there is an applet to show the output of the terminal. If we use "date +%T" or any other options, it would show the clock. If we add "-u" after 'date', it sould who UTC time. However, it seems to not be able to show times from other zones, meaning it does not seem to be able to execute the following command; "TZ='America/New_York'. It works in the terminal but not in the applet. If someone knows how to fix that, would be the best solution.
b) Other solution I found was the "FoxClocks" Google Chrome extension, with puts a strip above the taskbar with multiple option.
c) Perhaps the best option "MultiClock" @ https://extensions.gnome.org/extension/605/multiclock/
.... well, not sure how to configure that