public void setInternal(long secondsAsMillis, int nanos) {
    // This is our way of documenting that we are MUTATING the contents of
    // this writable's internal timestamp.
    updateTimestamp(timestamp, secondsAsMillis, nanos);

    bytesEmpty = true;
    timestampEmpty = false;
  }
true if data is stored in timestamp field rather than byte arrays.
     allows for lazy conversion to bytes when necessary
false otherwise
serde/src/java/org/apache/hadoop/hive/serde2/io/TimestampWritableV2.java