@Override
  protected long getTimestampField(TimestampColumnVector timestampColVector, int elementNum) {
    /* january is 0 */
    return 1 + super.getTimestampField(timestampColVector, elementNum);
  }
Returns month value.
Extends {@link VectorUDFTimestampFieldDate}
ql/src/java/org/apache/hadoop/hive/ql/exec/vector/expressions/VectorUDFMonthTimestamp.java