This Simple Desktop Clock GNOME Extension Looks Great on Ubuntu
If you want to add a clock to your Ubuntu desktop (or the desktop of any Linux distro using GNOME Shell) install the Desktop Clock GNOME extension.
This super simple, typographic timepiece puts the current time and date on your desktop.
Naturally, you can position the clock anywhere on your desktop, and tweak the typography to suit your tastes. It inherits your system font by default, and puts the time and date on separate lines, but you can choose to have it in a single line, and use any font that you want.
And what “you want” is the point here: this is a highly customisable widget for your desktop. You can pick a custom font, increase or decrease the font size, change font colour (including its opacity), and tweak the text shadow to make your timepiece pop (or set a totally transparent colour for no drop-shadow effect at all).