JSFiddle - React, Tailwind, and code Playground

HTML

<div class="span3" id="checkin">
    <span class="text-label"><i class="icon-calendar"></i>Check In</span> 
    <input type="text" name="checkin" value="02/08/2014" />
</div>

<div class="span3" id="checkout">
    <span class="text-label"><i class="icon-calendar"></i>Check Out</span>
    <input type="text" name="checkout" value="04/08/2014" />
</div>

<div class="span3" id="result">
    <span class="text-label"><i class="icon-calendar"></i>Duration</span>
    <input type="text" name="result" value="" />
</div>

JavaScript

$( document ).ready(function() {
    var start = $("input[name=checkin]").val().split("/");
    var end = $("input[name=checkout]").val().split("/");
    
    var startDate = new Date(start[2], start[1]-1, start[0]);
    var endDate = new Date(end[2], end[1]-1, end[0]);
    
    $("input[name=result]").val((endDate - startDate) / (1000*60*60*24));
});