JavaScript เปรียบเทียบวันที่ มากกว่า น้อยกว่า แสดงเตือนแบบ alert รบกวนด้วยนะค่ะ

<?
 $start_year = date("Y");
 $start_month = date("m");
 $start_day = date("d");
 
$end_year = date("Y");
$end_month = date("m");
$end_day = date("d");
 
 ?>
 <form method="post" onSubmit="return check_form(this)" >
วันเริ่ม
 <select name= "start_year" onChange="change_input(1,'')" id="id_year[1]"  >
	 <? for($y=date('Y')-2; $y<=date('Y')+2; $y++){?>
	<option value="<?=$y?>" <? if($y==$start_year){ echo "selected";}?>><?=$y+543?></option>
	<? } ?>
</select> -
<select name= "start_month" onChange="change_input(1,'')" id="id_month[1]">
	<? for($m=1; $m<=12; $m++){ if($m<10){$m="0".$m;}?>
	<option value="<?=$m?>" <? if($m==$start_month){ echo "selected";}?>><?=$m?></option>
	<? } ?>
</select> - 
<select name= "start_day"  id="id_day[1]" >													
</select>
วันสิ้นสุด
<select name= "end_year" onChange="change_input(2,'')" id="id_year[2]">
	 <? for($y=date('Y')-2; $y<=date('Y')+2; $y++){?>
	<option value="<?=$y?>" <? if($y==$end_year){ echo "selected";}?>><?=$y+543?></option>
	<? } ?>
</select> -
<select name= "end_month" onChange="change_input(2,'')" id="id_month[2]">
	<? for($m=1; $m<=12; $m++){ if($m<10){$m="0".$m;}?>
	<option value="<?=$m?>" <? if($m==$end_month){ echo "selected";}?>><?=$m?></option>
	<? } ?>
</select> - 
<select name= "end_day"  id="id_day[2]" >													
</select>
<input type="submit" />	
</form>
<script>

var d_start = <?=$start_day?>;
var d_end = <?=$end_day?>;
change_input(1,d_start);
change_input(2,d_end);

function change_input(i,day_selected)
{		

	year_id= document.getElementById('id_year['+i+']');
	month_id = document.getElementById('id_month['+i+']');
	day_id = document.getElementById('id_day['+i+']');
	
	if(year_id.value &&month_id.value)
		{
			calculator_day(year_id,month_id,day_id,day_selected);					
		}
}


function calculator_day(year_id,month_id,day_id,day_selected)
{
	var y_value = parseFloat(year_id.value);
	var m_value = parseFloat(month_id.value)-1;
	var start_month = new Date(y_value,m_value,1);
	var end_month = new Date(y_value,m_value+1,1);
	var num_day = (end_month - start_month)/1000/60/60/24;	
	show_select_day(num_day,day_id,day_selected);	
}


function show_select_day(num_day,day_id,day_selected)	 													
{	
	day_id.length = parseFloat(num_day)+1; 													
	for(var cn = 1; cn <= num_day; cn++)  																	
		{	 				
			day_id.options[cn].value = cn;	
			if(cn<10){cn="0"+cn}
			day_id.options[cn].text = cn;	
			if(day_selected==cn){day_id.options[cn].selected=true;}		
		}			
}	

function check_form(name_form)
{	 	
		
			var Y_start = name_form.start_year.value
			var m_start = name_form.start_month.value
			var d_start = name_form.start_day.value	
			var date_start = new Date(Y_start,(m_start-1),d_start);
			
			var Y_end = name_form.end_year.value
			var m_end= name_form.end_month.value
			var d_end = name_form.end_day.value	
			var date_end = new Date(Y_end,(m_end-1),d_end);
					
			if(date_start>date_end){
			alert("กรุณาเลือกวันสิ้นสุดมากกว่าวันเริ่ม")
			return false;
			
			}
						
	return confirm("ยืนยัน");	

}
</script>

<?php
	function compareDate($date1,$date2) {
		$arrDate1 = explode("-",$date1);
		$arrDate2 = explode("-",$date2);
		$timStmp1 = mktime(0,0,0,$arrDate1[1],$arrDate1[2],$arrDate1[0]);
		$timStmp2 = mktime(0,0,0,$arrDate2[1],$arrDate2[2],$arrDate2[0]);

		if ($timStmp1 == $timStmp2) {
			echo "\$date = \$date2";
		} else if ($timStmp1 > $timStmp2) {
			echo "\$date > \$date2";
		} else if ($timStmp1 < $timStmp2) {
			echo "\$date < \$date2";
		}
	}
	echo compareDate("2004-01-06","2004-05-06");
?>
ก็แค่เปลี่ยน จาก tag php ให้เป็น javascript บางส่วนสิครับ ก็ได้แล้ว เช่น 
<?php
	function compareDate($date1,$date2) {
		$arrDate1 = explode("-",$date1);
		$arrDate2 = explode("-",$date2);
		$timStmp1 = mktime(0,0,0,$arrDate1[1],$arrDate1[2],$arrDate1[0]);
		$timStmp2 = mktime(0,0,0,$arrDate2[1],$arrDate2[2],$arrDate2[0]);

		if ($timStmp1 == $timStmp2) {
			echo "<script>alert("วันที่เท่ากัน");</script>";
		} else if ($timStmp1 > $timStmp2) {
			echo "<script>alert("Date 1 มากกว่า Date 2");</script>";
		} else if ($timStmp1 < $timStmp2) {
			echo "<script>alert("Date 1 น้อยกว่า Date 2");</script>";
		}
	}
	echo compareDate("2004-01-06","2004-05-06");
?>