Calculate medium timestamps properly

h2/src/main/org/h2/expression/AggregateDataMedian.java

@@ -100,19 +100,33 @@ class AggregateDataMedian extends AggregateData {
         case Value.TIMESTAMP: {
             ValueTimestamp ts0 = (ValueTimestamp) v0.convertTo(Value.TIMESTAMP),
                     ts1 = (ValueTimestamp) v1.convertTo(Value.TIMESTAMP);
-            return ValueTimestamp.fromDateValueAndNanos(
-                    DateTimeUtils.dateValueFromAbsoluteDay((DateTimeUtils.absoluteDayFromDateValue(ts0.getDateValue())
-                            + DateTimeUtils.absoluteDayFromDateValue(ts1.getDateValue())) / 2),
-                    (ts0.getTimeNanos() + ts1.getTimeNanos()) / 2);
+            long dateSum = DateTimeUtils.absoluteDayFromDateValue(ts0.getDateValue())
+                    + DateTimeUtils.absoluteDayFromDateValue(ts1.getDateValue());
+            long nanos = (ts0.getTimeNanos() + ts1.getTimeNanos()) / 2;
+            if ((dateSum & 1) != 0) {
+                nanos += (DateTimeUtils.NANOS_PER_DAY / 2);
+                if (nanos >= DateTimeUtils.NANOS_PER_DAY) {
+                    nanos -= DateTimeUtils.NANOS_PER_DAY;
+                    dateSum++;
+                }
+            }
+            return ValueTimestamp.fromDateValueAndNanos(DateTimeUtils.dateValueFromAbsoluteDay(dateSum / 2), nanos);
         }
         case Value.TIMESTAMP_TZ: {
             ValueTimestampTimeZone ts0 = (ValueTimestampTimeZone) v0.convertTo(Value.TIMESTAMP_TZ),
                     ts1 = (ValueTimestampTimeZone) v1.convertTo(Value.TIMESTAMP_TZ);
-            return ValueTimestampTimeZone.fromDateValueAndNanos(
-                    DateTimeUtils.dateValueFromAbsoluteDay((DateTimeUtils.absoluteDayFromDateValue(ts0.getDateValue())
-                            + DateTimeUtils.absoluteDayFromDateValue(ts1.getDateValue())) / 2),
-                    (ts0.getTimeNanos() + ts1.getTimeNanos()) / 2,
-                    (short) ((ts0.getTimeZoneOffsetMins() + ts1.getTimeZoneOffsetMins()) / 2));
+            long dateSum = DateTimeUtils.absoluteDayFromDateValue(ts0.getDateValue())
+                    + DateTimeUtils.absoluteDayFromDateValue(ts1.getDateValue());
+            long nanos = (ts0.getTimeNanos() + ts1.getTimeNanos()) / 2;
+            if ((dateSum & 1) != 0) {
+                nanos += (DateTimeUtils.NANOS_PER_DAY / 2);
+                if (nanos >= DateTimeUtils.NANOS_PER_DAY) {
+                    nanos -= DateTimeUtils.NANOS_PER_DAY;
+                    dateSum++;
+                }
+            }
+            return ValueTimestampTimeZone.fromDateValueAndNanos(DateTimeUtils.dateValueFromAbsoluteDay(dateSum / 2),
+                    nanos, (short) ((ts0.getTimeZoneOffsetMins() + ts1.getTimeZoneOffsetMins()) / 2));
         }
         default:
             // Just return first

h2/src/main/org/h2/util/DateTimeUtils.java

@@ -42,7 +42,10 @@ public class DateTimeUtils {
      */
     public static final TimeZone UTC = TimeZone.getTimeZone("UTC");
 
-    private static final long NANOS_PER_DAY = MILLIS_PER_DAY * 1000000;
+    /**
+     * The number of nanoseconds per day.
+     */
+    public static final long NANOS_PER_DAY = MILLIS_PER_DAY * 1000000;
 
     private static final int SHIFT_YEAR = 9;
     private static final int SHIFT_MONTH = 5;

h2/src/test/org/h2/test/scripts/functions/aggregate/median.sql

@@ -584,6 +584,17 @@ select median(v) from test;
 > ---------------------
 > 2000-01-15 15:00:00.0
 
+delete from test;
+> update count: 5
+
+insert into test values ('2000-01-20 20:00:00'), ('2000-01-21 20:00:00');
+> update count: 2
+
+select median(v) from test;
+> MEDIAN(V)
+> ---------------------
+> 2000-01-21 08:00:00.0
+
 drop table test;
 > ok
 
@@ -619,5 +630,16 @@ select median(v) from test;
 > ------------------------
 > 2000-01-15 15:00:00.0+03
 
+delete from test;
+> update count: 5
+
+insert into test values ('2000-01-20 20:00:00+10'), ('2000-01-21 20:00:00-09');
+> update count: 2
+
+select median(v) from test;
+> MEDIAN(V)
+> ---------------------------
+> 2000-01-21 08:00:00.0+00:30
+
 drop table test;
 > ok