@Override
    public void reset(AggregationBuffer agg) throws HiveException {
      ((NumericStatsAgg)agg).reset("Date");
    }
GenericUDAFDateStatsEvaluator
High/low value will be saved in stats DB as long value representing days since epoch.
ql/src/java/org/apache/hadoop/hive/ql/udf/generic/GenericUDAFComputeStats.java