JSFiddle - React, Tailwind, and code Playground

HTML

<link rel="stylesheet" href="http://ajax.googleapis.com/ajax/libs/jqueryui/1.8.4/themes/ui-lightness/jquery-ui.css">

JavaScript

var start = new Date(2005,0,1),
    end = new Date(2021,0,1),
    today = new Date();

alert( Math.round(((end - start) * 100 ) / today) + '%' );