public HiveDecimal getHiveDecimal() {
    if (timestampEmpty) {
      populateTimestamp();
    }
    return getHiveDecimal(timestamp);
  }
Given an integer representing nanoseconds, write its serialized
value to the byte array b at offset

@param nanos
@param b
@param offset
@return
serde/src/java/org/apache/hadoop/hive/serde2/io/TimestampWritable.java