ต้องการให้โปรแกรม+ เพิ่มเดือนให้ จะต้องทำยังไงค่ะ รบกวนด้วยน่ะค่ะ

function totalmonth()
{	var stDate=new Date()
	var enDate= new Date()
	dd1=parseInt(document.getElementById("DD1").value)
	mm1=parseInt(document.getElementById("MM1").value)-1
	yy1=parseInt(document.getElementById("YY1").value)
	M1=parseInt(document.getElementById("Total").value)
	stDate.setFullYear(yy1,mm1,dd1)
	enDate.setFullYear(yy1,mm1,dd1+M1)
	document.getElementById("DD2").value=enDate.getDate();
	document.getElementById("MM2").value=enDate.getMonth()+1;
	document.getElementById("YY2").value=enDate.getFullYear();
}

function check_day()
{	var billdate=new Date()
	var enDate=new Date()
	var datediff=new Date()
	dd1=parseInt(document.form1.DD1.value)
	mm1=parseInt(document.form1.MM1.value)-1
	yy1=parseInt(document.form1.YY1.value)
	billdate.setFullYear(yy1,mm1,dd1) 
	dd2=parseInt(document.form1.DD2.value)
	mm2=parseInt(document.form1.MM2.value)-1
	yy2=parseInt(document.form1.YY2.value)
	enDate.setFullYear(yy2,mm2,dd2) 
	datediff=enDate-billdate // =second *1000
	total_day=datediff/24/3600/1000
	document.form1.Total.value=total_day;
}