I just wanted to see UTC time on my GNOME Shell desktop, just as mibus did.
Go to https://extensions.gnome.org/extension/1183/utcclock/ and turn on the extension.
git clone https://github.com/injcristianrojas/UTCClock.git
cd UTCClock
make install
Depending if you have Wayland or not, you may have to log out and then log back in.
This method is good if you don't want to wait for GNOME Extensions approval.
This extension works for GNOME versions from 45 up.
If you need a version that supports GNOME 3.22 to 3.38, switch to the
legacy branch. For GNOME 40 to 44, switch to the gnome_40_44 branch and do a manual install.