Vendor/phpspreadsheet/phpoffice/phpspreadsheet/src/PhpSpreadsheet/Writer/Xls/Workbook.php
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);
    }
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);
    }
Write DATEMODE record to indicate the date system in use (1904 or 1900).