I needed to see the current Unix Timestamp all the time, just as injcristianrojas and mibus for UTC.
Go to https://extensions.gnome.org/extension/1375/unix-timestamp-clock/ and turn on the extension.
cd ~/.local/share/gnome-shell/extensions/
git clone https://github.com/se1exin/unix-timestamp-clock.git [email protected]