Repair always-broken date_part('quarter',timestamp).

Previous result did not have correct month boundaries so anything near edge
 cases was suspect (e.g. April was in Q1 and July, August were lumped into
 Q2).
Thanks to Denis Osadchy <osadchy@turbo.nsk.su> for the report.

src/backend/utils/adt/timestamp.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $Header: /cvsroot/pgsql/src/backend/utils/adt/timestamp.c,v 1.40 2000/12/07 18:38:59 tgl Exp $
+ *	  $Header: /cvsroot/pgsql/src/backend/utils/adt/timestamp.c,v 1.41 2001/01/03 16:48:02 thomas Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -1887,9 +1887,9 @@ interval_trunc(PG_FUNCTION_ARGS)
 }
 
 /* isoweek2date()
- *
- * 	Convert ISO week of year number to date. An year must be already set.  
- *	karel 2000/08/07
+ * Convert ISO week of year number to date.
+ * The year field must be specified!
+ * karel 2000/08/07
  */
 void
 isoweek2date( int woy, int *year, int *mon, int *mday)
@@ -2053,7 +2053,7 @@ timestamp_part(PG_FUNCTION_ARGS)
 					break;
 
 				case DTK_QUARTER:
-					result = (tm->tm_mon / 4) + 1;
+					result = ((tm->tm_mon - 1) / 3) + 1;
 					break;
 
 				case DTK_WEEK: