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

doc/src/sgml/func.sgml

 <!--
-$PostgreSQL: pgsql/doc/src/sgml/func.sgml,v 1.309 2006/03/06 04:53:50 momjian Exp $
+$PostgreSQL: pgsql/doc/src/sgml/func.sgml,v 1.310 2006/03/06 22:49:15 momjian Exp $
 PostgreSQL documentation
 -->
 
@@ -5287,11 +5287,14 @@ SELECT SUBSTRING('XY1234Z', 'Y*?([0-9]{1,3})');
    <indexterm>
     <primary>isfinite</primary>
    </indexterm>
+   <indexterm>
+    <primary>justify_days</primary>
+   </indexterm>
    <indexterm>
     <primary>justify_hours</primary>
    </indexterm>
    <indexterm>
-    <primary>justify_days</primary>
+    <primary>justify_interval</primary>
    </indexterm>
    <indexterm>
     <primary>localtime</primary>
@@ -5429,6 +5432,14 @@ SELECT SUBSTRING('XY1234Z', 'Y*?([0-9]{1,3})');
         <entry><literal>true</literal></entry>
        </row>
 
+       <row>
+        <entry><literal><function>justify_days</function>(<type>interval</type>)</literal></entry>
+        <entry><type>interval</type></entry>
+        <entry>Adjust interval so 30-day time periods are represented as months</entry>
+        <entry><literal>justify_days(interval '30 days')</literal></entry>
+        <entry><literal>1 month</literal></entry>
+       </row>
+
        <row>
         <entry><literal><function>justify_hours</function>(<type>interval</type>)</literal></entry>
         <entry><type>interval</type></entry>
@@ -5438,11 +5449,11 @@ SELECT SUBSTRING('XY1234Z', 'Y*?([0-9]{1,3})');
        </row>
 
        <row>
-        <entry><literal><function>justify_days</function>(<type>interval</type>)</literal></entry>
+        <entry><literal><function>justify_interval</function>(<type>interval</type>)</literal></entry>
         <entry><type>interval</type></entry>
-        <entry>Adjust interval so 30-day time periods are represented as months</entry>
-        <entry><literal>justify_days(interval '30 days')</literal></entry>
-        <entry><literal>1 month</literal></entry>
+        <entry>Adjust interval using <function>justify_days</> and <function>justify_hours</>, with additional sign adjustments</></entry>
+        <entry><literal>justify_interval(interval '1 mon -1 hour')</literal></entry>
+        <entry><literal>29 days 23:00:00</literal></entry>
        </row>
 
        <row>
@@ -5486,13 +5497,6 @@ SELECT SUBSTRING('XY1234Z', 'Y*?([0-9]{1,3})');
      </tgroup>
     </table>
 
-   <para>
-    If you are using both <function>justify_hours</> and
-    <function>justify_days</>, it is best to use <function>justify_hours</>
-    first so any additional days will be included in the
-    <function>justify_days</> calculation.
-   </para>
-
    <para>
     In addition to these functions, the SQL <literal>OVERLAPS</> operator is
     supported:

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);
 }
 

src/include/catalog/catversion.h

@@ -37,7 +37,7 @@
  * Portions Copyright (c) 1996-2006, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/catalog/catversion.h,v 1.318 2006/03/05 15:58:54 momjian Exp $
+ * $PostgreSQL: pgsql/src/include/catalog/catversion.h,v 1.319 2006/03/06 22:49:16 momjian Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -53,6 +53,6 @@
  */
 
 /*							yyyymmddN */
-#define CATALOG_VERSION_NO	200602281
+#define CATALOG_VERSION_NO	200603061
 
 #endif

src/include/catalog/pg_proc.h

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2006, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/catalog/pg_proc.h,v 1.400 2006/03/05 15:58:54 momjian Exp $
+ * $PostgreSQL: pgsql/src/include/catalog/pg_proc.h,v 1.401 2006/03/06 22:49:16 momjian Exp $
  *
  * NOTES
  *	  The script catalog/genbki.sh reads this file and generates .bki
@@ -1462,6 +1462,8 @@ DATA(insert OID = 1173 (  timestamptz	   PGNSP PGUID 12 f f t f i 1 1184 "702" _
 DESCR("convert abstime to timestamp with time zone");
 DATA(insert OID = 1174 (  timestamptz	   PGNSP PGUID 12 f f t f s 1 1184 "1082" _null_ _null_ _null_	date_timestamptz - _null_ ));
 DESCR("convert date to timestamp with time zone");
+DATA(insert OID = 2711 (  justify_interval PGNSP PGUID 12 f f t f i 1 1186 "1186" _null_ _null_ _null_	interval_justify_interval - _null_ ));
+DESCR("promote groups of 24 hours to numbers of days and promote groups of 30 days to numbers of months");
 DATA(insert OID = 1175 (  justify_hours    PGNSP PGUID 12 f f t f i 1 1186 "1186" _null_ _null_ _null_	interval_justify_hours - _null_ ));
 DESCR("promote groups of 24 hours to numbers of days");
 DATA(insert OID = 1295 (  justify_days	   PGNSP PGUID 12 f f t f i 1 1186 "1186" _null_ _null_ _null_	interval_justify_days - _null_ ));

src/include/utils/timestamp.h

@@ -6,7 +6,7 @@
  * Portions Copyright (c) 1996-2006, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/utils/timestamp.h,v 1.58 2006/03/05 15:59:08 momjian Exp $
+ * $PostgreSQL: pgsql/src/include/utils/timestamp.h,v 1.59 2006/03/06 22:49:17 momjian Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -234,6 +234,7 @@ extern Datum interval_cmp(PG_FUNCTION_ARGS);
 extern Datum interval_hash(PG_FUNCTION_ARGS);
 extern Datum interval_smaller(PG_FUNCTION_ARGS);
 extern Datum interval_larger(PG_FUNCTION_ARGS);
+extern Datum interval_justify_interval(PG_FUNCTION_ARGS);
 extern Datum interval_justify_hours(PG_FUNCTION_ARGS);
 extern Datum interval_justify_days(PG_FUNCTION_ARGS);
 

src/test/regress/expected/interval.out

@@ -241,3 +241,10 @@ SELECT justify_days(interval '6 months 36 days 5 hours 4 minutes 3 seconds') as
  @ 7 mons 6 days 5 hours 4 mins 3 secs
 (1 row)
 
+-- test justify_interval()
+SELECT justify_interval(interval '1 month -1 hour') as "1 month -1 hour";
+  1 month -1 hour   
+--------------------
+ @ 29 days 23 hours
+(1 row)
+

src/test/regress/sql/interval.sql

@@ -76,3 +76,6 @@ select '4 millenniums 5 centuries 4 decades 1 year 4 months 4 days 17 minutes 31
 SELECT justify_hours(interval '6 months 3 days 52 hours 3 minutes 2 seconds') as "6 mons 5 days 4 hours 3 mins 2 seconds";
 SELECT justify_days(interval '6 months 36 days 5 hours 4 minutes 3 seconds') as "7 mons 6 days 5 hours 4 mins 3 seconds";
 
+-- test justify_interval()
+
+SELECT justify_interval(interval '1 month -1 hour') as "1 month -1 hour";