protected function _validateIsoDate($value)
    {
        // Test if value is blank.
        if(trim($value) != '')
        {
            // Test basic date format (yyyy-mm-dd)
            $pattern = '/^([0-9]{4})-([0-9]{2})-([0-9]{2})$/D';
            $return  = preg_match($pattern, $value, $matches) && checkdate($matches[2], $matches[3], $matches[1]);
        }
        else $return = false;

        return $return;
    }