/**
*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 <=0) {
	return false;
}

$m = date("m",$endtime);
$y = date("Y",$endtime);
$d = date("d",$endtime);
$year = floor($monthes/12);
$monthes_mod = $monthes%12;
if($year == 0){
//<12个月
	if($m < $monthes){
		//当前月份小于n月前
		$res_y = $y-1;
		$res_m = 12-($monthes-$m);
		$t = date("t",strtotime($res_y."-".$res_m."-01"));
		if($d<$t){
			$res_d = $d;
		}else{
			$res_d = $t;
		}
	}elseif($m == $monthes){
		$res_y = $y-1;
		$res_m = 12;
		$t = date("t",strtotime($res_y."-".$res_m."-01"));
		if($d<$t){
			$res_d = $d;
		}else{
			$res_d = $t;
		}
	}elseif($m>$monthes){
		$res_y = $y;
		$res_m = $m-$monthes;
		$t = date("t",strtotime($res_y."-".$res_m."-01"));
		if($d<$t){
			$res_d = $d;
		}else{
			$res_d = $t;
		}
	}
}elseif($year>0){
	$y = $y-$year;
	$monthes = $monthes_mod;
	if($m < $monthes){
		//当前月份小于n月前
		$res_y = $y-1;
		$res_m = 12-($monthes-$m);
		$t = date("t",strtotime($res_y."-".$res_m."-01"));
		if($d<$t){
			$res_d = $d;
		}else{
			$res_d = $t;
		}
	}elseif($m == $monthes){
		$res_y = $y-1;
		$res_m = 12;
		$t = date("t",strtotime($res_y."-".$res_m."-01"));
		if($d<$t){
			$res_d = $d;
		}else{
			$res_d = $t;
		}
	}elseif($m>$monthes){
		$res_y = $y;
		$res_m = $m-$monthes;
		$t = date("t",strtotime($res_y."-".$res_m."-01"));
		if($d<$t){
		$res_d = $d;
		}else{
			$res_d = $t;
		}
	}
}
return strtotime($res_y."-".$res_m."-".$res_d);
}



/**
*根据$endtime,返回指定$monthes月之前的日
*/
function severalMonthAgoEx($endtime,$monthes){
if (!$endtime) {
	return false;
}
if (!is_int($monthes) || $monthes <=0) {
	return false;
}

$m = date("m",$endtime);
$y = date("Y",$endtime);
$d = date("d",$endtime);

$year = floor($monthes/12);
$monthes_mod = $monthes%12;
$y -= $year;
//$year =0或>0;
//$monthes_mod =0或>0或
if ($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<$t){
	$res_d = $d;
}else{
	$res_d = $t;
}
return strtotime($res_y."-".$res_m."-".$res_d);
}

PHP实现指定时间的n月之前的这一天的两种算法,布布扣,bubuko.com

PHP实现指定时间的n月之前的这一天的两种算法