code: public static function PHPToExcel($dateValue)
      {
          if ((is_object($dateValue)) && ($dateValue instanceof DateTimeInterface)) {
              return self::dateTimeToExcel($dateValue);
          } elseif (is_numeric($dateValue)) {
              return self::timestampToExcel($dateValue);
          } elseif (is_string($dateValue)) {
              return self::stringToExcel($dateValue);
          }
  
          return false;
      }
docs: Convert a date from PHP to an MS Excel serialized date/time value.
  
  @param mixed $dateValue Unix Timestamp or PHP DateTime object or a string
  
  @return bool|float Excel date/time value
  or boolean FALSE on failure