I needed to see the current Unix Timestamp all the time, just as injcristianrojas and mibus for UTC.
Copy-paste currently shown timestamp (left mouse button) or current timestamp (any other mouse button) to clipboard.
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 unix-timestamp-clock@se1exin.github.com
- @Teddytrombone - Copy to clipboard, Gnome 3.38 fix