สอบถามการแสดงวันที่ให้เป็นค่าว่างและฟอร์มของวันที่่ครับ

<?php
	include("connect.php");
	
	$depart_id 		= $_POST['depart_id'];
	$com_user 		= $_POST['com_user'];
	$com_name 		= $_POST['com_name'];
	$com_address 	= $_POST['com_address'];
	$work_id		= $_POST['work_id'];
	$day 			= $_POST['day'];
	$month 			= $_POST['month'];
	$year 			= $_POST['year'];
	$com_cpu 		= $_POST['com_cpu'];
	$com_mb 		= $_POST['com_mb'];
	$com_ram 		= $_POST['com_ram'];
	$com_vga 		= $_POST['com_vga'];
	$hard_id 		= $_POST['hard_id'];
	$monitor_type 	= $_POST['monitor_type'];
	$monitor_name 	= $_POST['monitor_name'];
	$day2 			= $_POST['day2'];
	$month2 		= $_POST['month2'];
	$year2 			= $_POST['year2'];
	$day3 			= $_POST['day3'];
	$month3 		= $_POST['month3'];
	$year3 			= $_POST['year3'];	
	
	if(!checkdate($month,$day,$year))
	{
		echo "<script>alert('Pleae check date again!!!');history.back(-1);</script>";
		exit();
	}
	
	
	$com_date = "$year-$month-$day";
	$monitor_date = "$year2-$month2-$day2";
	$monitor_2 = "$year3-$month3-$day3";
	   
			  
	$sql = "insert into com values(null,'$depart_id','$com_user','$com_name','$com_address','$work_id','$com_date','$com_cpu','$com_mb',
	'$com_ram','$com_vga','$hard_id','$monitor_type','$monitor_name','$monitor_date','$monitor_2')";
	mysql_query($sql)or die("error=$sql");
	
	echo "<script type='text/javascript'>alert('Insert Complete!!!');window.location='show.php';</script>";
	
	mysql_close();
	
?>