JSFiddle - React, Tailwind, and code Playground

HTML

<link rel="stylesheet" href="//maxcdn.bootstrapcdn.com/twitter-bootstrap/2.3.2/css/bootstrap-combined.min.css">
<script src="//maxcdn.bootstrapcdn.com/twitter-bootstrap/2.3.2/js/bootstrap.min.js"></script>
<div class="progress progress-info">
  <div class="bar"></div>
</div>

JavaScript

var start = new Date(2015,0,1), // Jan 1, 2015
    end = new Date(2015,7,24), // June 24, 2015
    today = new Date(); // April 23, 2015

var startDate = start.getTime();
var endDate = end.getTime();
var todayDate = today.getTime();

// Get the total possible timestamp value
var total = endDate - startDate;

// Get the current value
var current = todayDate - startDate;

// Get the percentage
var p = Math.round((current / total) * 100)  + '%';

$('.bar').css( "width", p ).after().append(p);