Does not work on GNOME Shell 3.36.4

[Cat-Phoenix]

gnome-shell --version
GNOME Shell 3.36.4

Nothing on top bar, nothing on tweaks tools (tweaks -> extensions -> nothing like multiclock)

[ivan-pinatti]

I have the same version and it works partially for me.
It is show on the top bar, but when I click on "other" to change the timezone, nothing happens. I can only select the ones that are shown in the drop-down list.

[mibus]

The current version now supports 3.36 (should definitely fix ivan-pinatti's issue). I've uploaded a new version to extensions.gnome.org but it'll take a while to get reviewed before it's available for download there too.

Cat-Phoenix, it seems likely that the install didn't happen. Can you check that it's installed to ~/.local/share/gnome-shell/extensions/MultiClock@mibus.org/ ?