Adding A month to the entered date using javasript

try this code. its working well :-)

<script type="text/javascript">
$(function () {
$('#date2').focus(function () {
	            var istDateVal = $("#date1").val().split("/");
	            var istDate = new Date();
	            istDate.setFullYear(istDateVal[2], istDateVal[1] - 1, istDateVal[0]);
	            istDate.setMonth(istDate.getMonth() - 1);
	            $('#date2').val(istDate.getDate() + "/" + (istDate.getMonth() + 1) + "/" + istDate.getFullYear())
	        });
});
</script> 

<div>
    <input type="text" date1=" /><br mode=" hold=" />    <input type=" text="" id="date2" />
</div>

function populateArchive()
{
var frm = document.form1;
var input;
var monthVal;
var monthMax = new Array(31, 31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31);
var top;


if (validateDateFormat(frm.story_intro_date) && frm.story_exp_date.disabled == false)
{

var post_date = new Date(frm.story_intro_date.value);
var expiration_date = new Date(post_date.setMonth(post_date.getMonth()));
var new_date = expiration_date.getDate();
var new_month = expiration_date.getMonth() + 2;
var new_year = expiration_date.getFullYear();
if (new_month == 13)
{
new_month = new_month - 12;
new_year = new_year + 1;
}
input = parseInt(new_date, 10);
monthVal = new_month;
top = monthMax[monthVal];
if (!inRange(input, 1, top))
{
expiration_date_new = new_month + "/" + top + "/" + new_year;
}
else
expiration_date_new = new_month + "/" + new_date + "/" + new_year;
frm.story_exp_date.value = expiration_date_new;
frm.hdn_story_exp_date.value = expiration_date_new;
}

}




I faced problem in getting the desired result in the second text box if I had entered 12th month in the first text box so to face that problem i have included an if block and checked whether it is in range and added an year with that and reduced 11 months from it so that it gives the result as the first month of next year :)