In my mind there were two categories of open issues

  a) ones that are 100% backward (such as the comment about
     outputting this format)
and
  b) ones that aren't (such as deprecating the current
     postgresql shorthand of
         '1Y1M'::interval = 1 year 1 minute
     in favor of the ISO-8601
         'P1Y1M'::interval = 1 year 1 month.

Attached is a patch that addressed all the discussed issues that
did not break backward compatability, including the ability to
output ISO-8601 compliant intervals by setting datestyle to
iso8601basic.

Interval values can now be written as  ISO 8601 time intervals, using
the "Format with time-unit designators". This format always starts with
the character 'P', followed  by a string of values followed
by single character time-unit designators. A 'T' separates the date and
time parts of the interval.

Ron Mayer

doc/src/sgml/datatype.sgml

 <!--
-$PostgreSQL: pgsql/doc/src/sgml/datatype.sgml,v 1.135 2003/12/01 22:07:55 momjian Exp $
+$PostgreSQL: pgsql/doc/src/sgml/datatype.sgml,v 1.136 2003/12/20 15:32:54 momjian Exp $
 -->
 
  <chapter id="datatype">
@@ -1785,6 +1785,57 @@ January 8 04:05:06 1999 PST
       <replaceable>p</replaceable> should be between 0 and 6, and
       defaults to the precision of the input literal.
      </para>
+
+
+     <para>
+      Alternatively, <type>interval</type> values can be written as 
+      ISO 8601 time intervals, using the "Format with time-unit designators".
+      This format always starts with the character <literal>'P'</>, followed 
+      by a string of values followed by single character time-unit designators.
+      A <literal>'T'</> separates the date and time parts of the interval.
+     </para>
+
+     <para>
+       Format:  PnYnMnDTnHnMnS
+     </para>
+     <para>
+       In this format, <literal>'n'</> gets replaced by a number, and 
+       <literal>Y</> represents years, 
+       <literal>M</> (in the date part) months,
+       <literal>D</> months,
+       <literal>H</> hours,
+       <literal>M</> (in the time part) minutes,
+       and <literal>S</> seconds.
+     </para>
+      
+
+     <table id="interval-example-table">
+	   <title>Interval Example</title>
+	   <tgroup cols="2">
+		<thead>
+		 <row>
+		  <entry>Traditional</entry>
+		  <entry>ISO-8601 time-interval</entry>
+		 </row>
+		</thead>
+		<tbody>
+		 <row>
+		  <entry>1 month</entry>
+		  <entry>P1M</entry>
+		 </row>
+		 <row>
+		  <entry>1 hour 30 minutes</entry>
+		  <entry>PT1H30M</entry>
+		 </row>
+		 <row>
+		  <entry>2 years 10 months 15 days 10 hours 30 minutes 20 seconds</entry>
+		  <entry>P2Y10M15DT10H30M20S</entry>
+		 </row>
+		</tbody>
+	   </thead>
+	  </table>
+	  
+     </para>
     </sect3>
 
     <sect3>
@@ -1941,6 +1992,11 @@ January 8 04:05:06 1999 PST
          <entry>regional style</entry>
          <entry>17.12.1997 07:37:16.00 PST</entry>
         </row>
+	<row>
+	 <entry>ISO8601basic</entry>
+	 <entry>ISO 8601 basic format</entry>
+	 <entry>19971217T073716-08</entry>
+	</row>
        </tbody>
       </tgroup>
      </table>
@@ -1997,6 +2053,11 @@ January 8 04:05:06 1999 PST
 </programlisting>
     </para>
 
+    <para>
+	 If the <varname>datestyle</> is set to iso8601basic, the interval
+	 output is a ISO-8601 time interval with time-unit designator (like P1Y6M or PT23H59M59S).
+    </para>
+
     <para>
      The date/time styles can be selected by the user using the
      <command>SET datestyle</command> command, the

src/backend/commands/variable.c

@@ -9,7 +9,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/commands/variable.c,v 1.90 2003/11/29 19:51:48 pgsql Exp $
+ *	  $PostgreSQL: pgsql/src/backend/commands/variable.c,v 1.91 2003/12/20 15:32:54 momjian Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -82,7 +82,12 @@ assign_datestyle(const char *value, bool doit, bool interactive)
 
 		/* Ugh. Somebody ought to write a table driven version -- mjl */
 
-		if (strcasecmp(tok, "ISO") == 0)
+		if (strcasecmp(tok, "ISO8601BASIC") == 0)
+		{
+			newDateStyle = USE_ISO8601BASIC_DATES;
+			scnt++;
+		}
+		else if (strcasecmp(tok, "ISO") == 0)
 		{
 			newDateStyle = USE_ISO_DATES;
 			scnt++;
@@ -198,6 +203,9 @@ assign_datestyle(const char *value, bool doit, bool interactive)
 		case USE_ISO_DATES:
 			strcpy(result, "ISO");
 			break;
+		case USE_ISO8601BASIC_DATES:
+			strcpy(result, "ISO8601BASIC");
+			break;
 		case USE_SQL_DATES:
 			strcpy(result, "SQL");
 			break;

src/include/miscadmin.h

@@ -12,7 +12,7 @@
  * Portions Copyright (c) 1996-2003, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/miscadmin.h,v 1.138 2003/11/29 22:40:53 pgsql Exp $
+ * $PostgreSQL: pgsql/src/include/miscadmin.h,v 1.139 2003/12/20 15:32:55 momjian Exp $
  *
  * NOTES
  *	  some of the information in this file should be moved to
@@ -150,6 +150,8 @@ extern DLLIMPORT Oid MyDatabaseId;
  *	USE_ISO_DATES specifies ISO-compliant format
  *	USE_SQL_DATES specifies Oracle/Ingres-compliant format
  *	USE_GERMAN_DATES specifies German-style dd.mm/yyyy
+ *	USE_ISO8601BASIC_DATES specifies ISO-8601-basic format (including
+ *                         ISO compliant but non-human-friendly intervals)
  *
  * DateOrder defines the field order to be assumed when reading an
  * ambiguous date (anything not in YYYY-MM-DD format, with a four-digit
@@ -169,6 +171,7 @@ extern DLLIMPORT Oid MyDatabaseId;
 #define USE_ISO_DATES			1
 #define USE_SQL_DATES			2
 #define USE_GERMAN_DATES		3
+#define USE_ISO8601BASIC_DATES	4
 
 /* valid DateOrder values */
 #define DATEORDER_YMD			0

src/interfaces/ecpg/pgtypeslib/dt.h

@@ -21,6 +21,7 @@ typedef double fsec_t;
 #define USE_ISO_DATES					1
 #define USE_SQL_DATES					2
 #define USE_GERMAN_DATES				3
+#define USE_ISO8601BASIC_DATES			4
 
 #define DAGO			"ago"
 #define EPOCH			"epoch"

src/interfaces/ecpg/pgtypeslib/dt_common.c

@@ -704,6 +704,16 @@ EncodeDateOnly(struct tm * tm, int style, char *str, bool EuroDates)
 					  -(tm->tm_year - 1), tm->tm_mon, tm->tm_mday, "BC");
 			break;
 
+		case USE_ISO8601BASIC_DATES:
+			/* compatible with ISO date formats */
+			if (tm->tm_year > 0)
+				sprintf(str, "%04d%02d%02d",
+						tm->tm_year, tm->tm_mon, tm->tm_mday);
+			else
+				sprintf(str, "%04d%02d%02d %s",
+					  -(tm->tm_year - 1), tm->tm_mon, tm->tm_mday, "BC");
+			break;
+
 		case USE_SQL_DATES:
 			/* compatible with Oracle/Ingres date formats */
 			if (EuroDates)
@@ -820,6 +830,51 @@ EncodeDateTime(struct tm * tm, fsec_t fsec, int *tzp, char **tzn, int style, cha
 			}
 			break;
 
+		case USE_ISO8601BASIC_DATES:
+			/* Compatible with ISO-8601 date formats */
+
+			sprintf(str, "%04d%02d%02dT%02d%02d",
+				  ((tm->tm_year > 0) ? tm->tm_year : -(tm->tm_year - 1)),
+					tm->tm_mon, tm->tm_mday, tm->tm_hour, tm->tm_min);
+
+			/*
+			 * Print fractional seconds if any.  The field widths here
+			 * should be at least equal to MAX_TIMESTAMP_PRECISION.
+			 *
+			 * In float mode, don't print fractional seconds before 1 AD,
+			 * since it's unlikely there's any precision left ...
+			 */
+#ifdef HAVE_INT64_TIMESTAMP
+			if (fsec != 0)
+			{
+				sprintf((str + strlen(str)), "%02d.%06d", tm->tm_sec, fsec);
+#else
+			if ((fsec != 0) && (tm->tm_year > 0))
+			{
+				sprintf((str + strlen(str)), "%09.6f", tm->tm_sec + fsec);
+#endif
+				TrimTrailingZeros(str);
+			}
+			else
+				sprintf((str + strlen(str)), "%02d", tm->tm_sec);
+
+			if (tm->tm_year <= 0)
+				sprintf((str + strlen(str)), " BC");
+
+			/*
+			 * tzp == NULL indicates that we don't want *any* time zone
+			 * info in the output string. *tzn != NULL indicates that we
+			 * have alpha time zone info available. tm_isdst != -1
+			 * indicates that we have a valid time zone translation.
+			 */
+			if ((tzp != NULL) && (tm->tm_isdst >= 0))
+			{
+				hour = -(*tzp / 3600);
+				min = ((abs(*tzp) / 60) % 60);
+				sprintf((str + strlen(str)), ((min != 0) ? "%+03d:%02d" : "%+03d"), hour, min);
+			}
+			break;
+
 		case USE_SQL_DATES:
 			/* Compatible with Oracle/Ingres date formats */
 

src/interfaces/ecpg/pgtypeslib/interval.c

@@ -442,6 +442,17 @@ DecodeInterval(char **field, int *ftype, int nf, int *dtype, struct tm * tm, fse
 	return (fmask != 0) ? 0 : -1;
 }	/* DecodeInterval() */
 
+
+/* 
+ * Small helper function to avoid cut&paste in EncodeInterval below
+ */
+static char * AppendISO8601Fragment(char * cp, int value, char character) 
+{
+    sprintf(cp,"%d%c",value,character);
+    return cp + strlen(cp);
+}
+
+
 /* EncodeInterval()
  * Interpret time structure as a delta time and convert to string.
  *
@@ -449,6 +460,14 @@ DecodeInterval(char **field, int *ftype, int nf, int *dtype, struct tm * tm, fse
  * Actually, afaik ISO does not address time interval formatting,
  *	but this looks similar to the spec for absolute date/time.
  * - thomas 1998-04-30
+ * 
+ * Actually, afaik, ISO 8601 does specify formats for "time
+ * intervals...[of the]...format with time-unit designators", which
+ * are pretty ugly.  The format looks something like
+ *     P1Y1M1DT1H1M1.12345S
+ * If you want this (perhaps for interoperability with computers
+ * rather than humans), datestyle 'iso8601basic' will output these.
+ * - ron 2003-07-14
  */
 int
 EncodeInterval(struct tm * tm, fsec_t fsec, int style, char *str)
@@ -465,7 +484,12 @@ EncodeInterval(struct tm * tm, fsec_t fsec, int style, char *str)
 	 */
 	switch (style)
 	{
-			/* compatible with ISO date formats */
+			/* compatible with ISO date formats 
+			   ([ram] Not for ISO 8601, perhaps some other ISO format.
+			   but I'm leaving it that way because it's more human
+			   readable than ISO8601 time intervals and for backwards
+			   compatability.)
+			*/
 		case USE_ISO_DATES:
 			if (tm->tm_year != 0)
 			{
@@ -533,6 +557,48 @@ EncodeInterval(struct tm * tm, fsec_t fsec, int style, char *str)
 			}
 			break;
 
+		case USE_ISO8601BASIC_DATES:
+			sprintf(cp,"P");
+			cp++;
+			if (tm->tm_year != 0) cp = AppendISO8601Fragment(cp,tm->tm_year,'Y');
+			if (tm->tm_mon  != 0) cp = AppendISO8601Fragment(cp,tm->tm_mon ,'M');
+			if (tm->tm_mday != 0) cp = AppendISO8601Fragment(cp,tm->tm_mday,'D');
+			if ((tm->tm_hour != 0) || (tm->tm_min != 0) ||
+				(tm->tm_sec  != 0) || (fsec       != 0))
+			{
+				sprintf(cp,"T"),
+				cp++;
+			}
+			if (tm->tm_hour != 0) cp = AppendISO8601Fragment(cp,tm->tm_hour,'H');
+			if (tm->tm_min  != 0) cp = AppendISO8601Fragment(cp,tm->tm_min ,'M');
+
+			if ((tm->tm_year == 0) && (tm->tm_mon == 0) && (tm->tm_mday == 0) &&
+				(tm->tm_hour == 0) && (tm->tm_min == 0) && (tm->tm_sec  == 0) &&
+				(fsec        == 0))
+            {
+				sprintf(cp,"T0S"),
+				cp+=2;
+            }
+            else if (fsec != 0)
+            {
+#ifdef HAVE_INT64_TIMESTAMP
+				sprintf(cp, "%d", abs(tm->tm_sec));
+				cp += strlen(cp);
+				sprintf(cp, ".%6dS", ((fsec >= 0) ? fsec : -(fsec)));
+#else
+				fsec += tm->tm_sec;
+				sprintf(cp, "%fS", fabs(fsec));
+#endif
+				TrimTrailingZeros(cp);
+				cp += strlen(cp);
+			}
+			else if (tm->tm_sec != 0)
+			{
+				cp = AppendISO8601Fragment(cp,tm->tm_sec ,'S');
+				cp += strlen(cp);
+			}
+			break;
+
 		case USE_POSTGRES_DATES:
 		default:
 			strcpy(cp, "@ ");