php strtotime เรียนถามเรื่อง strtotime('-1 month')) ครับ

<?php
function add_date($givendate,$day=0,$mth=0,$yr=0) {
		$cd = strtotime($givendate);
		$newdate = date('Y-m-d h:i:s', mktime(date('h',$cd),
		date('i',$cd), date('s',$cd), date('m',$cd)+$mth,
		date('d',$cd)+$day, date('Y',$cd)+$yr));
		return $newdate;
     }

echo add_date("2010-12-09",2,1,0);
?>

<?
$inputDate = "2011-09-09 15:25:40";
$strCurrDate = strtotime($inputDate);
echo date("Y-m-d H:i:s", mktime(date("H",$strCurrDate)+5, date("i",$strCurrDate)+0, date("s",$strCurrDate)+0, date("m",$strCurrDate)+0  , date("d",$strCurrDate)+0, date("Y",$strCurrDate)+0));
?>