code: private function readDateMode()
      {
          $length = self::getUInt2d($this->data, $this->pos + 2);
          $recordData = $this->readRecordData($this->data, $this->pos + 4, $length);
  
          // move stream pointer to next record
          $this->pos += 4 + $length;
  
          // offset: 0; size: 2; 0 = base 1900, 1 = base 1904
          Date::setExcelCalendar(Date::CALENDAR_WINDOWS_1900);
          if (ord($recordData[0]) == 1) {
              Date::setExcelCalendar(Date::CALENDAR_MAC_1904);
          }
      }
docs: DATEMODE.
  
  This record specifies the base date for displaying date
  values. All dates are stored as count of days past this
  base date. In BIFF2-BIFF4 this record is part of the
  Calculation Settings Block. In BIFF5-BIFF8 it is
  stored in the Workbook Globals Substream.
  
  --    "OpenOffice.org's Documentation of the Microsoft
  Excel File Format"