private function format_d(&$str) {
    if (strstr($str, '%d'))
      $str = str_replace('%d', sprintf('%02d', $this->day), $str);
  }
private function format_d(&$str) {
    if (strstr($str, '%d'))
      $str = str_replace('%d', sprintf('%02d', $this->day), $str);
  }
Day of the month, 2 digits with leading zeros
@param string $str