$ sudo apt-get update
$ sudo apt-get install libjs-jquery-countdown 

libjs-jquery-countdown is installed in your system.

Make ensure the libjs-jquery-countdown package were installed using the commands given below,

$ sudo dpkg-query -l | grep libjs-jquery-countdown *

You will get with libjs-jquery-countdown package name, version, architecture and description in a table

libjs-jquery-countdown package basic information: