ถามแนวคิด เมื่อทำการเลือกวันที่เริ่มต้น แล้วต้องการให้แสดงวันสิ้นสุด คะ

<script type="text/javascript">
function Pad(input, pad_length, pad_string, pad_type) {
    var half = '', pad_to_go;
    var pad_repeater = function (s, len) {
        var collect = '', i;
        while (collect.length < len) {collect += s;}
        collect = collect.substr(0,len);

        return collect;
    };
    input += '';
    pad_string = pad_string !== undefined ? pad_string : ' ';
    if (pad_type != 'STR_PAD_LEFT' && pad_type != 'STR_PAD_RIGHT' && pad_type != 'STR_PAD_BOTH') { pad_type = 'STR_PAD_RIGHT'; }
    if ((pad_to_go = pad_length - input.length) > 0) {
        if (pad_type == 'STR_PAD_LEFT') { input = pad_repeater(pad_string, pad_to_go) + input; }
        else if (pad_type == 'STR_PAD_RIGHT') { input = input + pad_repeater(pad_string, pad_to_go); }
        else if (pad_type == 'STR_PAD_BOTH') {
            half = pad_repeater(pad_string, Math.ceil(pad_to_go/2));
            input = half + input + half;
            input = input.substr(0, pad_length);
        }
        
    }
    return input;
}

function calcEndDate(frm){
	var objDate =new Date();
	var setRangeVal = frm.range_num.value;
	var setRangeType = frm.range_type.options[frm.range_type.selectedIndex].value;
	var setStartDate = frm.start_date.value;
	var objEndDate = frm.end_date;
	if(setRangeVal != "" && setStartDate != ""){
		var dateSplited = setStartDate.split('/');
		if(dateSplited.length == 3){
			objDate.setFullYear(dateSplited[2], (parseInt(dateSplited[1])-1), dateSplited[0]);
			
			if(setRangeType == "Days"){
				objDate.setDate(objDate.getDate()+parseInt(setRangeVal));
			}else if(setRangeType == "Months"){
				objDate.setMonth(objDate.getMonth()+parseInt(setRangeVal));
			}else if(setRangeType == "Years"){
				objDate.setFullYear(objDate.getFullYear()+parseInt(setRangeVal));
			}
			
			if(!isNaN(objDate.getDate()) && !isNaN(objDate.getMonth()) && !isNaN(objDate.getFullYear())){
				objEndDate.value = objDate.getDate() + '/' + Pad((objDate.getMonth()+1), 2, '0', 'STR_PAD_LEFT') + '/' + objDate.getFullYear();
			}
		}
	}
}
</script>

<form id="form1" name="form1" method="post" action="">
  <input type="text" name="range_num" id="range_num" size="2" onkeyup="javascript:calcEndDate(this.form);" />
  <select name="range_type" id="range_type" onchange="javascript:calcEndDate(this.form);">
  <option value="Days" selected="selected">Days</option>
  <option value="Months">Months</option>
  <option value="Years">Years</option>
  </select>
  <br />
  <input type="text" name="start_date" id="start_date" onkeyup="javascript:calcEndDate(this.form);" />
  <br />
  <input type="text" name="end_date" id="end_date" readonly="readonly" />
</form>