php oop สงสัยโค้ด ทำตามตัวอย่างแล้วรันมา มันแสดงผลไม่ตรงกันครับ

error_reporting(E_ALL);
class thai_datetime{
	var $datetime;
	function thai_datetime($valuedate = NULL) {
		$this->set_datetime($valuedate);
	}
	function set_datetime($valuedate) {
		if(empty($valuedate))
			$this->datetime = date("Y-m-d H:i:s");
		else
			$this->datetime = $valuedate;
	}
	function cast_datetime($value, $type) {
		$value = strtotime($value);
		switch($type) {
			case "d": return date("j", $value);
			case "m": return date("m", $value);
			case "y": return date("y", $value);
			case "Y": return date("Y", $value);
			case "H": return date("H", $value);
			case "M": return date("M", $value);
			case "S": return date("S", $value);
		}
	}
	function short_month($m_val = NULL) {
		if(empty($m_val))
			$m_val = $this->cast_datetime($this->datetime,"m");
		switch($m_val) {
			case 1: return "ม.ค."; break;
			case 2: return "ก.พ."; break;
			case 3: return "มี.ค."; break;
			case 4: return "เม.ย."; break;
			case 5: return "พ.ค."; break;
			case 6: return "มิ.ย."; break;
			case 7: return "ก.ค."; break;
			case 8: return "ส.ค."; break;
			case 9: return "ก.ย."; break;
			case 10: return "ต.ค."; break;
			case 11: return "พ.ย"; break;
			case 12: return "ธ.ค."; break;
			default: return "ม.ค."; break;
		}
	}
	function long_month($m_val = NULL) {
		if(empty($m_val))
			$m_val = $this->cast_datetime($this->datetime,"m");
		switch($m_val) {
			case 1: return "มกราคม"; break;
			case 2: return "กุมภาพันธ์"; break;
			case 3: return "มีนาคม"; break;
			case 4: return "เมษายน"; break;
			case 5: return "พฤษภาคม"; break;
			case 6: return "มิถุนายน"; break;
			case 7: return "กรกฏาคม"; break;
			case 8: return "สิงหาคม"; break;
			case 9: return "กันยายน"; break;
			case 10: return "ตุลาคม"; break;
			case 11: return "พฤศจิกายน"; break;
			case 12: return "ธันวาคม"; break;
			default: return "มกราคม"; break;
		}
	}
	function year($value = NULL) {
		if(empty($value))
			return $this->cast_datetime($this->datetime,"Y")+543;
		else return $value+543;
	}
	function month() {
		return $this->cast_datetime($this->datetime,"m");
	}
	function day() {
		return $this->cast_datetime($this->datetime,"d");
	}
	function hour() {
		return $this->cast_datetime($this->datetime,"H");
	}
	function minute() {
		return $this->cast_datetime($this->datetime,"M");
	}
	function second() {
		return $this->cast_datetime($this->datetime,"S");
	}
}

$x = new thai_datetime();
echo $x->day() , " " , $x->short_month(), " " ,$x->year();
echo '<br />';
$y = new thai_datetime("2005-1-20");
echo $y->day() , " " , $y->short_month(), " " ,$y->year();

case "Y": return date("Y", $value);

return $this->cast_datetime($this->datetime,"Y")+543;