JSFiddle - React, Tailwind, and code Playground

HTML

<div id="bat"><div id="fill"></div></div>

CSS

#bat {
    width:100px;
    height: 30px;
    border: 1px solid;
    position: relative;
}
#fill {
    height: 100%;
    background-color: red;
    width: 0%;
}

JavaScript

var now = new Date();
var today = now.getTime();
var pastDate = Date.parse("Tue, 1 Jan 2012 00:00:00 GMT");
var futureDate = Date.parse("Tue, 1 Aug 2012 00:00:00 GMT");
var count = Math.floor((today-pastDate)/(futureDate-pastDate)*100);

if(count > 0) { //move Loading bar by percent 
    $('#fill').animate({
       width: count+"%"
    }, 1000);
}