[php] 날짜 계산

<?
$date = date("Y-m-d");

$_3day_ago  = date("Y-m-d" , strtotime($date."-3 days") );
//3달전
$_3month_ago  = date("Y-m-d" , strtotime($date."-3 month") );
// 3일 후 : 
$_3day_later  = date("Y-m-d" , strtotime($date."+3 days") );  

?>