You need to create an empty paragraph on the page with the ID of 'demo'. Put it where you want the date to appear. The script will populate that paragraph.
In the script you find commented how to set the time and date you want to count down to. It has to be entered in the same format as the one currently shown in the script. Towards the end is a place where you can write some text for when the specified date has arrived.
Use this script for something you want to keep track of, like the wedding date, when a new baby is due, retirement, as a few examples.