Vendor/phpspreadsheet/phpoffice/phpspreadsheet/src/PhpSpreadsheet/Reader/Xls.php
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);
        }
    }
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);
        }
    }
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"