Cara, achei essa aqui no link http://www.php.net/manual/en/ref.datetime.php function sub($timestamp, $seconds, $minutes, $hours, $days, $months, $years) {
$timePieces = getdate($timestamp);
return mktime( $timePieces["hours"] - $hours,
$timePieces["minutes"] - $minutes,
$timePieces["seconds"] - $seconds,
$timePieces["mon"] - $months,
$timePieces["mday"] - $days,
$timePieces["year"] - $years );
} Não testei, mas creio que funciona