public void set(TimestampWritable t) {
    if (t.bytesEmpty) {
      set(t.getTimestamp());
      return;
    }
    if (t.currentBytes == t.externalBytes) {
      set(t.currentBytes, t.offset);
    } else {
      set(t.currentBytes, 0);
    }
  }
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/TimestampWritable.java