Vendor/phpspreadsheet/phpoffice/phpspreadsheet/src/PhpSpreadsheet/Shared/Date.php
public static function excelToTimestamp($excelTimestamp, $timeZone = null)
    {
        return (int) self::excelToDateTimeObject($excelTimestamp, $timeZone)
            ->format('U');
    }
public static function excelToTimestamp($excelTimestamp, $timeZone = null)
    {
        return (int) self::excelToDateTimeObject($excelTimestamp, $timeZone)
            ->format('U');
    }
Convert a MS serialized datetime value from Excel to a unix timestamp.

@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 int Unix timetamp for this date/time