PHP实现指定时间的n月之前的这一天的两种算法

/***根据$endtime,返回指定$monthes月之前的日*/function severalMonthAgo($endtime,$monthes){if (!$endtime) {	return false;}if (!is_int($monthes) || $monthes $monthes){		$res_y = $y;		$res_m = $m-$monthes;		$t = date("t",strtotime($res_y."-".$res_m."-01"));		if($d0){	$y = $y-$year;	$monthes = $monthes_mod;	if($m $monthes){		$res_y = $y;		$res_m = $m-$monthes;		$t = date("t",strtotime($res_y."-".$res_m."-01"));		if($d0;//$monthes_mod =0或>0或if ($year>0) {	if ($monthes_mod>0) {		$m_diff = 12+$m-$monthes_mod;		$m_diff_div = floor($m_diff/12);		$m_diff_mod = $m_diff%12;		if ($m_diff_div > 0) {			$res_y = $y;			$res_m = $m_diff_mod;		}elseif ($m_diff_div == 0) {			$res_y = $y -1;			$res_m = $m_diff_mod;		}	}elseif ($monthes_mod==0) {		//整除		$res_y = $y;		$res_m = $m;			}}elseif ($year == 0) {	$m_diff = 12+$m-$monthes_mod;	$m_diff_div = floor($m_diff/12);	$m_diff_mod = $m_diff%12;	if ($m_diff_div > 0) {		$res_y = $y;	}elseif ($m_diff_div == 0) {		$res_y = $y -1;	}	$res_m = $m_diff_mod;}$t = date("t",strtotime($res_y."-".$res_m."-01"));if($d  <br>