ขอ JavaScript DateAdd/DateDiff ลบวันที่ไป 30 วันหน่อยค่ะ ช่วยหน่อยนะค่ะ

<script language="javascript">
function fncDateadd(){
var _amt = parseInt(document.getElementById("amount").value);

var d = new Date();
d.setDate(d.getDate()+_amt);

mkMonth=d.getMonth()+1;
mkMonth=new String(mkMonth);
if(mkMonth.length==1){
mkMonth="0"+mkMonth;
}
mkDay=d.getDate();
mkDay=new String(mkDay);
if(mkDay.length==1){
mkDay="0"+mkDay;
}	
mkYear=d.getFullYear();
document.getElementById("txtdate").value = mkDay+"-"+mkMonth+"-"+mkYear;
}
</script>


<input name="amount" type="text" size="1" />
<input type="text" name="txtdate" id="txtdate" />
<input type="button" value="check" onclick="fncDateadd();" />

var datetime =new Date();
datetime.setDate(datetime.getDate()-30);