def modification_time(self):
    """dfdatetime.DateTimeValues: modification time or None if not available."""
    if self._stat_info is None:
      return None

    timestamp = int(self._stat_info.st_mtime)
    return dfdatetime_posix_time.PosixTime(timestamp=timestamp)
dfdatetime.DateTimeValues: modification time or None if not available.