A simple gnome shell extension to add a countdown in the top left corner of the screen. It can be configured with a custom date by simply clicking on it. It will open a popup calendar prompting you to enter a date.
Simply run these commands:
cd ~/.local/share/gnome-shell/extensions
git clone https://github.com/mdev9/gnome-simple-countdown.git
mv gnome-simple-countdown simple_countdown@mdev9/
gnome-extensions enable simple_countdown@mdev9