@Override
  public String toString() {
    if (timestampEmpty) {
      populateTimestamp();
    }

    if (timestamp.getNanos() > 0) {
      return timestamp.toString();
    }

    return timestamp.toLocalDateTime().format(DATE_TIME_FORMAT);
  }
@return double representation of the timestamp, accurate to nanoseconds
serde/src/java/org/apache/hadoop/hive/serde2/io/TimestampWritable.java