I know this is not a “World Clock” but for general event coordination, even across multiple venues, http://time.is is a useful tool. We use it for multi-campus coordination of live linking.
Rather than relying on iPads and people phones (which always seem to go flat) to display the time, we can use a Raspberry Pi running a browser in Kiosk mode to generate an accurate clock and feed this into our video hub for distribution.
Overview of setting up your Raspberry Pi
There are many resources on the Internet for performing these tasks (like https://www.raspberrypi.org/forums/viewtopic.php?t=163316) so I won’t go into the detail here but the basic steps are:
Install noob onto SD card as per instructions
I did do the standard update to ensure the latest stuff was installed.
The default seems to be to boot into the graphical UI.
I changed the /boot/config.txt to force the HDMI mode to match my ATEM video standard of 720p 50Hz
From the desktop, run a terminal session and from that run leafpad.
Press down arrow to get the path box to display and type
(this is a user file /home/pi/.config/lxsession/LXDE-pi/autostart, not the one you find in /etc/ )
Add the following to the end of the file and then save.
@xset s off
@chromium-browser –disable-session-crashed-bubble –disable-infobars –kiosk http://time.is/just
Other options that may be usefull
@chromium-browser –noerrdialogs –incognito
You can experiment with the Time.is display options to change font and color, then update the url for Chromium.