Vendor/phpspreadsheet/phpoffice/phpspreadsheet/src/PhpSpreadsheet/Shared/Date.php
public static function setExcelCalendar($baseDate)
    {
        if (($baseDate == self::CALENDAR_WINDOWS_1900) ||
            ($baseDate == self::CALENDAR_MAC_1904)) {
            self::$excelCalendar = $baseDate;

            return true;
        }

        return false;
    }
public static function setExcelCalendar($baseDate)
    {
        if (($baseDate == self::CALENDAR_WINDOWS_1900) ||
            ($baseDate == self::CALENDAR_MAC_1904)) {
            self::$excelCalendar = $baseDate;

            return true;
        }

        return false;
    }
Set the Excel calendar (Windows 1900 or Mac 1904).

@param int $baseDate Excel base date (1900 or 1904)

@return bool Success or failure