บวกวันที่แบบอัตโนมัติในinput text ช่องที่สองคับ แนะนำด้วยครับ

<script language="JavaScript">
var HttPRequest = false;
 
function doCallAjax(fProductID,fProductName) {
HttPRequest = false;
if (window.XMLHttpRequest) { // Mozilla, Safari,...
HttPRequest = new XMLHttpRequest();
if (HttPRequest.overrideMimeType) {
HttPRequest.overrideMimeType('text/html');
}
} else if (window.ActiveXObject) { // IE
try {
HttPRequest = new ActiveXObject("Msxml2.XMLHTTP");
} catch (e) {
try {
HttPRequest = new ActiveXObject("Microsoft.XMLHTTP");
} catch (e) {}
}
}
 
if (!HttPRequest) {
alert('Cannot create XMLHTTP instance');
return false;
}
 
var url = 'plus30day.php'; //ไฟล์+date 30 วัน
var pmeters = "tProductID=" + encodeURI( document.getElementById(fProductID).value);
 
HttPRequest.open('POST',url,true);
 
HttPRequest.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
HttPRequest.setRequestHeader("Content-length", pmeters.length);
HttPRequest.setRequestHeader("Connection", "close");
HttPRequest.send(pmeters);
 
 
HttPRequest.onreadystatechange = function()
{
 
 
if(HttPRequest.readyState == 4) // Return Request
{
var myProduct = HttPRequest.responseText;
if(myProduct != "")
{
/*var myArr = myProduct.split("|");*/
document.getElementById(fProductName).value = myProduct;
/*document.getElementById(fProductName).value = myArr[0];
document.getElementById(fPrice).value = myArr[1];*/
}
}
 
}

}
</script>

<?
print <<< HTML
<table>
<tr><td aling='right'>date1 ::</td><td><input type="text" name="min_date" id="min_date"  value='$min_date'   size="10" OnChange="JavaScript:doCallAjax('min_date','due_date');" onclick="popUpCalendar(this, b.min_date, 'dd/mm/yyyy')">
<img src="../../js/cal.gif" onclick="popUpCalendar(this, b.min_date, 'yyyy-mm-dd')" align="top" width="16" height="16">
</td> </tr>
<tr><td aling='right'>text2 ::</td><td><input type="text" name="due_date" id="due_date"  value='$due_date' size="10"  readonly OnMouseOver="JavaScript:doCallAjax('min_date','due_date');">
</td> </tr>
</table>
HTML;
?>

<?php
$strProduct = trim($_POST["tProductID"]);

$adddate=30;

Ymd2date($strProduct,$adddate);

return;
 

  
function Ymd2date($date1,$adddate) {

list($y1,$m1,$d1) = split("-",$date1);
$d1=(int)$d1;$m1=(int)$m1;$y1=(int)$y1;


$checkdate = mktime(0, 0, 0, $m1, $d1+$adddate, $y1);

echo date("Y-m-d",$checkdate);

return;


}; 

?>