The code seems to be OK. Try:
$now = date("Y-m-d h:i:s");
echo strtotime($now).'<br>';
echo strtotime('10-11-2012, 16:00:00');
and check the server time.
UPDATE:
Replace:
$sessionHTML .= sprintf("<option value='%s' class='%s'>%s - %s - %s</option>", $dbSessionId, $class, $dbSessionName, date("d-m-Y",strtotime($dbSessionDate)), date("H:i",strtotime($dbSessionTime))) . PHP_EOL;
with:
$sessionHTML .= sprintf("<option value='%s' style='color: %s'>%s - %s - %s</option>", $dbSessionId, $class, $dbSessionName, date("d-m-Y",strtotime($dbSessionDate)), date("H:i",strtotime($dbSessionTime))) . PHP_EOL;
to prevent css mistake.
Or replace:
strtotime($now) > strtotime($dbSessionDate)
with:
strtotime($now) < strtotime($dbSessionDate)