Puedes hacerlo tambien de esta forma sacado de los comentarios de
php.net Código PHP:
$time = mktime(0, 0, 0, 06, 30, 2009) - time();
$days = floor($time/86400);
$hours = floor(($time-($days*86400))/3600);
$mins = floor (($time-($days*86400)-($hours*3600))/60);
$secs = floor ($time-($days*86400)-($hours*3600)-($mins*60));
$tsecs = $time;
$thours = round($time/3600);
$tdays = round($thours/24);
echo $tdays;