code: public static function excelToTimestamp($excelTimestamp, $timeZone = null)
      {
          return (int) self::excelToDateTimeObject($excelTimestamp, $timeZone)
              ->format('U');
      }
docs: 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