Php 将月份添加到现有日期？

我有一个日期，需要加上几个月，但不知怎的，它返回了纪元日期

$duration = 28;
$row['end_date'] = '2010-09-22 0000:00:00';

$newEndDate =  date("Y-m-d", strtotime(" +".$duration." month",substr($row['end_date'],0,10)));

改用这个：

$date = date("Y-m-d");// current date

$date = strtotime(date("Y-m-d", strtotime($date)) . " +1 day");
$date = strtotime(date("Y-m-d", strtotime($date)) . " +1 week");
$date = strtotime(date("Y-m-d", strtotime($date)) . " +2 week");
$date = strtotime(date("Y-m-d", strtotime($date)) . " +1 month");
$date = strtotime(date("Y-m-d", strtotime($date)) . " +30 days");

改用这个：

$date = date("Y-m-d");// current date

$date = strtotime(date("Y-m-d", strtotime($date)) . " +1 day");
$date = strtotime(date("Y-m-d", strtotime($date)) . " +1 week");
$date = strtotime(date("Y-m-d", strtotime($date)) . " +2 week");
$date = strtotime(date("Y-m-d", strtotime($date)) . " +1 month");
$date = strtotime(date("Y-m-d", strtotime($date)) . " +30 days");