JS works in Codepen and JSfiddle but not in actual browswer

I made this super simple website to count down time since a certain date. I am not familiar with java script at all and copied the JavaScript and altered it to suite my needs.

As the title says it works in JSfiddle and Codepen but I can’t get it to work in my browser through Brackets.

Could someone take a look at the code and see if there are any glaring mistakes?

Here is the JSfiddle