.neT: Date difference in javascript

<!DOCTYPE html>
<html>
<head>
<script type="text/javascript">
function displayDate()
{
var days = 0;
var difference = 0;
var today=new Date('02/21/1021'); //CONVERT STRING TO DATE
var today1=new Date('02/18/1021'); //CONVERT STRING TO DATE
difference = today1- today;
days = Math.round(difference/(1000*60*60*24));//DIFFERENCE
alert(days)
}
</script>
</head>
<body>

<h1>My First Web Page</h1>
<p id="demo">This is a paragraph.</p>

<button type="button" onclick="displayDate()">Display Date</button>

</body>
</html>