public function validate($value)
 {
     $is_valid = true;
     if ($value) {
         $rule = new Rule_Date();
         $is_valid = $rule->isValid($value);
         if (!$is_valid) {
             $GLOBALS['Response']->addFeedback('error', $GLOBALS['Language']->getText('plugin_tracker_common_artifact', 'error_date_value', array($this->field->getLabel())));
         }
     }
     return $is_valid;
 }