Drupal 6 & CCK field filling manually

If you should fill CCK field manually you must fill 'value' field and '_error_element' field. Otherwise data will not be accepted.

/**
* Implementation of hook_nodeapi().
*/
function custom_nodeapi(&$node, $op) { 
 switch ($op) {   
  case 'presave':
   if ($node->type == 'order') {
    $node->field_date_creation[0]['value'] = format_date(time());
    $node->field_date_creation[0]['_error_element'] = 'field_date_creation][0][value';
   }
   break;
 }
}