Finding Difference Between Dates

Code:
<script language = 'JavaScript'>
<!--
function checkDate()
{ var d1, d2
d1 = new Date(document.form1.date1.value)
d2 = new Date(document.form1.date2.value)

if (d2.getTime() < d1.getTime()) {
window.alert ("The end date can not be before the start date.")
return false } else { return true }
}
//-->
</script>
, where date2 is the end date and date1 is the start date.

& then I have put
PHP Code:

<form name='form1' action="<?=$_SERVER['PHP_SELF']?>" ONSUBMIT = 'return checkDate()' method="post">

Each time I am doing submit, it is directly going to the alert, without checking the condition?