Install cairo-dock-clock-plug-in by entering the following commands in the terminal:
sudo apt update sudo apt install cairo-dock-clock-plug-in
Clock plug-in for Cairo-dock
A collection of official plug-ins and applets for cairo-dock. . This plug-in displays time and date in your dock. Two views are available : numeric and analogic, based on Cairo-Clock. This is compatible with the Cairo-Clock's themes, and you can detach itself to be a perfect clone of Cairo-Clock. And this supports alarms, and a basic calendar, and allows you to set time and date.