code: public long getModificationTime() {
      if (attributes.containsKey(FILE_MODIFICATION_TIMESTAMP_KEY)
          && attributes.get(FILE_MODIFICATION_TIMESTAMP_KEY) != null) {
        try {
          return Longs.fromByteArray(attributes.get(FILE_MODIFICATION_TIMESTAMP_KEY));
        } catch (IllegalArgumentException iae) {
          logger.atFine().log(
              "Failed to parse modification time '%s' millis for object %s",
              attributes.get(FILE_MODIFICATION_TIMESTAMP_KEY), itemInfo.getObjectName());
        }
      }
      return getCreationTime();
    }
docs: Gets the modification time of this file if one is set, otherwise the value of
  {@link #getCreationTime()} is returned.
  
  Time is expressed as milliseconds since January 1, 1970 UTC.