การคำนวนวันที่อ่ะค่ะ คือ จะเอาโค้ดที่มีการคำนวณวันที่แบบอัตโนมัติอ่ะค่ เช่น เลือกวันเริ่มลาวันที่ 24/04/2552

<?php
function DateDiff($strDate1,$strDate2)
{
return (strtotime($strDate2) - strtotime($strDate1))/ ( 60 * 60 * 24 ); // 1 day = 60*60*24
}
function TimeDiff($strTime1,$strTime2)
{
return (strtotime($strTime2) - strtotime($strTime1))/ ( 60 * 60 ); // 1 Hour = 60*60
}
function DateTimeDiff($strDateTime1,$strDateTime2)
{
return (strtotime($strDateTime2) - strtotime($strDateTime1))/ ( 60 * 60 ); // 1 Hour = 60*60
}

echo "Date Diff = ".DateDiff("2008-08-01","2008-08-31")."<br>";
echo "Time Diff = ".TimeDiff("00:00","19:00")."<br>";
echo "Date Time Diff = ".DateTimeDiff("2008-08-01 00:00","2008-08-01 19:00")."<br>";
?>

var stringToDateObject = function(stringDate) {
	var arrString = stringDate.split("/"); 
	return new Date(arrString[1] + "/" + arrString[0] + "/" + arrString[2]);
}

var dateBetween = function(sDate, eDate) {
	var startDate = stringToDateObject(sDate);
	var endDate   = stringToDateObject(eDate);
	var oneDay    = 24 * 60 * 60 * 1000;
	
	return Math.round(endDate.getTime() - startDate.getTime()) / oneDay;
}