Attached is the new patch. To summarize:

- new function justify_interval(interval)
   - modified function justify_hours(interval)
   - modified function justify_days(interval)

These functions are defined to meet the requirements as discussed in
this thread.  Specifically:

   - justify_hours makes certain the sign bit on the hours
     matches the sign bit on the days.  It only checks the
     sign bit on the days, and not the months, when
     determining if the hours should be positive or negative.
     After the call, -24 < hours < 24.

   - justify_days makes certain the sign bit on the days
     matches the sign bit on the months.  It's behavior does
     not depend on the hours, nor does it modify the hours.
     After the call, -30 < days < 30.

   - justify_interval makes sure the sign bits on all three
     fields months, days, and hours are all the same.  After
     the call, -24 < hours < 24 AND -30 < days < 30.

Mark Dilger

src/backend/utils/adt/timestamp.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/utils/adt/timestamp.c,v 1.161 2006/03/05 15:58:44 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/utils/adt/timestamp.c,v 1.162 2006/03/06 22:49:16 momjian Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -1974,6 +1974,82 @@ timestamp_mi(PG_FUNCTION_ARGS)
 	PG_RETURN_INTERVAL_P(result);
 }
 
+/*
+ *  interval_justify_interval()
+ *
+ *  Adjust interval so 'month', 'day', and 'time' portions are within
+ *  customary bounds.  Specifically:
+ *
+ *  	0 <= abs(time) < 24 hours
+ *  	0 <= abs(day)  < 30 days
+ *
+ *  Also, the sign bit on all three fields is made equal, so either
+ *  all three fields are negative or all are positive.
+ */
+Datum
+interval_justify_interval(PG_FUNCTION_ARGS)
+{
+	Interval   *span = PG_GETARG_INTERVAL_P(0);
+	Interval   *result;
+	
+#ifdef HAVE_INT64_TIMESTAMP
+	int64		wholeday;
+#else
+	double		wholeday;
+#endif
+	int32		wholemonth;
+
+	result = (Interval *) palloc(sizeof(Interval));
+	result->month = span->month;
+	result->day = span->day;
+	result->time = span->time;
+
+#ifdef HAVE_INT64_TIMESTAMP
+	TMODULO(result->time, wholeday, USECS_PER_DAY);
+#else
+	TMODULO(result->time, wholeday, (double) SECS_PER_DAY);
+#endif
+	result->day += wholeday;	/* could overflow... */
+
+	wholemonth = result->day / DAYS_PER_MONTH;
+	result->day -= wholemonth * DAYS_PER_MONTH;
+	result->month += wholemonth;
+
+	if (result->month > 0 &&
+		(result->day < 0 || (result->day == 0 && result->time < 0)))
+	{
+		result->day += DAYS_PER_MONTH;
+		result->month--;
+	}
+	else if (result->month < 0 &&
+		(result->day > 0 || (result->day == 0 && result->time > 0)))
+	{
+		result->day -= DAYS_PER_MONTH;
+		result->month++;
+	}
+
+	if (result->day > 0 && result->time < 0)
+	{
+#ifdef HAVE_INT64_TIMESTAMP
+		result->time += USECS_PER_DAY;
+#else
+		result->time += (double) SECS_PER_DAY;
+#endif
+		result->day--;
+	}
+	else if (result->day < 0 && result->time > 0)
+ 	{
+#ifdef HAVE_INT64_TIMESTAMP
+		result->time -= USECS_PER_DAY;
+#else
+		result->time -= (double) SECS_PER_DAY;
+#endif
+		result->day++;
+	}
+
+	PG_RETURN_INTERVAL_P(result);
+}
+
 /*
  *	interval_justify_hours()
  *
@@ -2006,6 +2082,25 @@ interval_justify_hours(PG_FUNCTION_ARGS)
 #endif
 	result->day += wholeday;	/* could overflow... */
 
+	if (result->day > 0 && result->time < 0)
+	{
+#ifdef HAVE_INT64_TIMESTAMP
+		result->time += USECS_PER_DAY;
+#else
+		result->time += (double) SECS_PER_DAY;
+#endif
+		result->day--;
+	}
+	else if (result->day < 0 && result->time > 0)
+	{
+#ifdef HAVE_INT64_TIMESTAMP
+		result->time -= USECS_PER_DAY;
+#else
+		result->time -= (double) SECS_PER_DAY;
+#endif
+		result->day++;
+	}
+
 	PG_RETURN_INTERVAL_P(result);
 }
 
@@ -2031,6 +2126,17 @@ interval_justify_days(PG_FUNCTION_ARGS)
 	result->day -= wholemonth * DAYS_PER_MONTH;
 	result->month += wholemonth;
 
+	if (result->month > 0 && result->day < 0)
+	{
+		result->day += DAYS_PER_MONTH;
+		result->month--;
+	}
+	else if (result->month < 0 && result->day > 0)
+	{
+		result->day -= DAYS_PER_MONTH;
+		result->month++;
+	}
+
 	PG_RETURN_INTERVAL_P(result);
 }