public function format($value)
  {
    if ($value===null) return null;

    $match = preg_match('/^(\d{4})-(\d{1,2})-(\d{1,2})$/', $value, $parts);
    $valid = ($match && checkdate((int)$parts[2], (int)$parts[3], (int)$parts[1]));
    if ($valid)
    {
      if ($value===$this->openDate) return '';

      $date = new \DateTime($value);

      return $date->format($this->format);
    }

    return $value;
  }