code: private function writeDateMode()
      {
          $record = 0x0022; // Record identifier
          $length = 0x0002; // Bytes to follow
  
          $f1904 = (Date::getExcelCalendar() == Date::CALENDAR_MAC_1904)
              ? 1
              : 0; // Flag for 1904 date system
  
          $header = pack('vv', $record, $length);
          $data = pack('v', $f1904);
          $this->append($header . $data);
      }
docs: Write DATEMODE record to indicate the date system in use (1904 or 1900).