a problem in theme function

hi every body when i tried to theme my form in table and tried the fowlloing example
function supervisors_display(){
$r = db_rewrite_sql(db_query('SELECT nid, title, created FROM {node} LIMIT 20'));
while ($row = db_fetch_object($r)) {
$rows[] = $row;
}

foreach ($rows as $v) {
$nids[$v->nid] = '';
$form['title'][$v->nid] = array('#type' => 'markup', '#value' => check_plain($v->title));
$form['created'][$v->nid] = array('#type' => 'markup', '#value' => date("d/m/Y H:i", $v->created));
}
$form['nids'] = array('#type' => 'checkboxes', '#options' => $nids);
$form['submit'] = array('#type' => 'submit', '#value' => 'Update');

$form['header'] = array(
'#type' => 'value',
'#value' => array(
array('data' => 'nid'),
array('data' => t('Title')),
array('data' => t('Created')),
)
);

return $form;
}

function theme_supervisors_display($form) {

foreach (element_children($form['title']) as $key) {
$row = array();
$row['data'][0] = form_render($form['nids'][$key]);
$row['data'][1] = form_render($form['title'][$key]);
$row['data'][2] = form_render($form['created'][$key]);
$rows[] = $row;
}

$output = form_render(theme('table', $form['header']['#value'], $rows));
$output.= form_render($form); // Process any other fields and display them
return $output;
}

actually the data appear to me like this

first03/12/2008 16:44

whith out any theme
so please if any one can help me please .