Vendor/phpspreadsheet/phpoffice/phpspreadsheet/src/PhpSpreadsheet/Shared/Date.php
public static function excelToDateTimeObject($excelTimestamp, $timeZone = null)
    {
        $timeZone = ($timeZone === null) ? self::getDefaultTimezone() : self::validateTimeZone($timeZone);
        if (Functions::getCompatibilityMode() == Functions::COMPATIBILITY_EXCEL) {
            if ($excelTimestamp < 1.0) {
                // Unix timestamp base date
                $baseDate = new \DateTime('1970-01-01', $timeZone);
            } else {
                // MS Excel calendar base dates
                if (self::$excelCalendar == self::CALENDAR_WINDOWS_1900) {
                    // Allow adjustment for 1900 Leap Year in MS Excel
                    $baseDate = ($excelTimestamp < 60) ? new \DateTime('1899-12-31', $timeZone) : new \DateTime('1899-12-30', $timeZone);
                } else {
                    $baseDate = new \DateTime('1904-01-01', $timeZone);
                }
            }
        } else {
            $baseDate = new \DateTime('1899-12-30', $timeZone);
        }

        $days = floor($excelTimestamp);
        $partDay = $excelTimestamp - $days;
        $hours = floor($partDay * 24);
        $partDay = $partDay * 24 - $hours;
        $minutes = floor($partDay * 60);
        $partDay = $partDay * 60 - $minutes;
        $seconds = round($partDay * 60);

        if ($days >= 0) {
            $days = '+' . $days;
        }
        $interval = $days . ' days';

        return $baseDate->modify($interval)
            ->setTime($hours, $minutes, $seconds);
    }
public static function excelToDateTimeObject($excelTimestamp, $timeZone = null)
    {
        $timeZone = ($timeZone === null) ? self::getDefaultTimezone() : self::validateTimeZone($timeZone);
        if (Functions::getCompatibilityMode() == Functions::COMPATIBILITY_EXCEL) {
            if ($excelTimestamp < 1.0) {
                // Unix timestamp base date
                $baseDate = new \DateTime('1970-01-01', $timeZone);
            } else {
                // MS Excel calendar base dates
                if (self::$excelCalendar == self::CALENDAR_WINDOWS_1900) {
                    // Allow adjustment for 1900 Leap Year in MS Excel
                    $baseDate = ($excelTimestamp < 60) ? new \DateTime('1899-12-31', $timeZone) : new \DateTime('1899-12-30', $timeZone);
                } else {
                    $baseDate = new \DateTime('1904-01-01', $timeZone);
                }
            }
        } else {
            $baseDate = new \DateTime('1899-12-30', $timeZone);
        }

        $days = floor($excelTimestamp);
        $partDay = $excelTimestamp - $days;
        $hours = floor($partDay * 24);
        $partDay = $partDay * 24 - $hours;
        $minutes = floor($partDay * 60);
        $partDay = $partDay * 60 - $minutes;
        $seconds = round($partDay * 60);

        if ($days >= 0) {
            $days = '+' . $days;
        }
        $interval = $days . ' days';

        return $baseDate->modify($interval)
            ->setTime($hours, $minutes, $seconds);
    }
Convert a MS serialized datetime value from Excel to a PHP Date/Time object.

@param float|int $excelTimestamp MS Excel serialized date/time value
@param null|DateTimeZone|string $timeZone The timezone to assume for the Excel timestamp,
if you don't want to treat it as a UTC value
Use the default (UST) unless you absolutely need a conversion

@throws \Exception

@return \DateTime PHP date/time object