Date picker dates Validation

This script validates the start and end dates that are selected from a datepicker.
Note that here, the dates are manually taken, but in real time, you get the input values of date1 and date2 ($a and $b in php code) from a date picker. Just use a datepicker and populate the values in input fields.

Script code:

<script type="text/javascript">
function check_dates()
{
var a = document.validate_dates.date1.value;
var b = document.validate_dates.date2.value;

// alert(a);
// alert(b);

a1 = a.split("-");
b1 = b.split("-");

// alert(a1);
// alert(a1.length);

var d1 = new Date();
d1.setFullYear(a1[0],a1[1]-1,a1[2]);

var d2 = new Date();
d2.setFullYear(b1[0],b1[1]-1,b1[2]);

// alert(d1);
// alert(d2);

if(d2 < d1)
{
alert("End date cannot be less than start date");
return false;
}
return true;
}
</script>

Form code:

<?php
$a = "2010-08-02"; // get it from date-picker
$b = "2010-08-01"; // get it from date-picker
?>
<form name="validate_dates" method="post" action="">
<table align="left" cellpadding="2" cellpadding="2">
<tr>
<td>Start Date:</td><td><input type="text" name="date1" value="<?php echo $a; ?>" disabled="disabled" /></td>
</tr>
<tr>
<td>End Date:</td><td><input type="text" name="date2" value="<?php echo $b; ?>" disabled="disabled" /></td>
</tr>
<tr>
<td colspan="2"><input type="button" name="validate" value="Validate Dates" onclick="return check_dates();" /></td>
</tr>
</table>
</form>