@Override
  public boolean matches(String name, ColumnVector.Type inputColVectorType,
      ColumnVector.Type outputColVectorType, Mode mode) {

    /*
     * Sum input TIMESTAMP and output DOUBLE.
     *
     * Just modes (PARTIAL1, COMPLETE).
     */
    return
        name.equals("sum") &&
        inputColVectorType == ColumnVector.Type.TIMESTAMP &&
        outputColVectorType == ColumnVector.Type.DOUBLE &&
        (mode == Mode.PARTIAL1 || mode == Mode.COMPLETE);
  }
Value is explicitly (re)initialized in reset()
ql/src/java/org/apache/hadoop/hive/ql/exec/vector/expressions/aggregates/VectorUDAFSumTimestamp.java