$realtime = "2010-01-01 12:00:00";

$mongotime = new Mongodate(strtotime($realtime));

$date  = $item['pubdate'];
(etc ...)
$unix_now = time();
$result = strtotime($date, $unix_now);
$unix_diff_min = (($unix_now  - $result) / 60);
$min = round($unix_diff_min);