Problem solved...I created a string in view.html.php as:
$item->start_date_obj = JFactory::getDate($item->start_date);
$item->finish_date_obj = JFactory::getDate($item->finish_date);
if ($item->start_date != '00:00'){
$item->start_date = JFactory::getDate($item->start_date)->format("H:i");
} else{
$item->start_date = JText::_('COM_SEMINARMAN_NOT_SPECIFIED');
}
if ($item->finish_date != '00:00'){
$item->finish_date = JFactory::getDate($item->finish_date)->format("H:i");
}else{
$item->finish_date = JText::_('COM_SEMINARMAN_NOT_SPECIFIED');
}
Thanks anyway