สอบถามบวก เพิ่ม วัน ระหว่าง วันที่ใน DB บวก กับ จำนวนวัน ครับ

<script>
  function calculate(aws,Condition_date,Condition_due_date)
{ 
var value1 = parseFloat(document.getElementById(aws).value); 
var value2 = parseFloat(document.getElementById(Condition_date).value);
var value3 = ""; 
if(value1 && value2){
value3 = value1+value2;
}
document.getElementById(Condition_due_date).value = value3;
} 
</script>


<input name="stock_date"  id="aws" onKeyPress="if(event.keyCode>=48 && event.keyCode<=57){}else{event.returnValue = false;}"  onkeyup="calculate('aws','Condition_date','Condition_due_date');" value="<?=$stock_date?>"/> 
<input name="Condition_date"   id="Condition_date" onKeyPress="if(event.keyCode>=48 && event.keyCode<=57){}else{event.returnValue = false;}"  onkeyup="calculate('aws','Condition_date','Condition_due_date');" value="<?=$day;?>"/> 
<input name="Condition_due_date"   id="Condition_due_date"  readonly="" />

    <?php
$signup=$stock_date; // 2013-11-05 
$singup_ar_=explode("-",$signup);
//echo $singup_ar_[2]."<br>";
//echo $singup_ar_[1]."<br>";
//echo $singup_ar_[0]."<br>";
$singup_ar_new="$singup_ar_[2]-$singup_ar_[1]-$singup_ar_[0]";// แปลงเป็น  05-11-2013
$day=$compa_creadite;// 30 day
$singup_ar=explode("-",$singup_ar_new);
//echo $singup_ar[0]."<br>";
//echo $singup_ar[1]."<br>";
//echo $singup_ar[2]."<br>";
$c_s=mktime(0,0,0,$singup_ar[1],$singup_ar[0]+$day,$singup_ar[2]);
$pay=date("Y-m-d",$c_s);//บวกกันแล้ว ได้ผลลัพธ์ 2013-12-05
$pay_cut=explode("-",$pay);
echo $newdate="$pay_cut[2]-$pay_cut[1]-$pay_cut[0]"; // ผลลัพธ์  5-12-2013
?>