diff --git a/src/backend/utils/adt/timestamp.c b/src/backend/utils/adt/timestamp.c
index 67e0cf9..723c670 100644
--- a/src/backend/utils/adt/timestamp.c
+++ b/src/backend/utils/adt/timestamp.c
@@ -27,6 +27,7 @@
 #include "funcapi.h"
 #include "libpq/pqformat.h"
 #include "miscadmin.h"
+#include "nodes/makefuncs.h"
 #include "nodes/nodeFuncs.h"
 #include "parser/scansup.h"
 #include "utils/array.h"
@@ -4874,6 +4875,87 @@ interval_part(PG_FUNCTION_ARGS)
 }
 
 
+/* timestamp_zone_transform()
+ * If the zone argument of a timestamp_zone() or timestamptz_zone() call is a
+ * plan-time constant denoting a zone equivalent to UTC, the call will always
+ * return its second argument unchanged.  Simplify the expression tree
+ * accordingly.  Civil time zones almost never qualify, because jurisdictions
+ * that follow UTC today have not done so continuously.
+ */
+Datum
+timestamp_zone_transform(PG_FUNCTION_ARGS)
+{
+	Node	   *func_node = (Node *) PG_GETARG_POINTER(0);
+	FuncExpr   *expr = (FuncExpr *) func_node;
+	Node	   *ret = NULL;
+	Node	   *zone_node;
+
+	Assert(IsA(expr, FuncExpr));
+	Assert(list_length(expr->args) == 2);
+
+	zone_node = (Node *) linitial(expr->args);
+
+	if (IsA(zone_node, Const) &&!((Const *) zone_node)->constisnull)
+	{
+		text	   *zone = DatumGetTextPP(((Const *) zone_node)->constvalue);
+		char		tzname[TZ_STRLEN_MAX + 1];
+		char	   *lowzone;
+		int			type,
+					abbrev_offset;
+		pg_tz	   *tzp;
+		bool		noop = false;
+
+		/*
+		 * If the timezone is forever UTC+0, the FuncExpr function call is a
+		 * no-op for all possible timestamps.  This passage mirrors code in
+		 * timestamp_zone().
+		 */
+		text_to_cstring_buffer(zone, tzname, sizeof(tzname));
+		lowzone = downcase_truncate_identifier(tzname,
+											   strlen(tzname),
+											   false);
+		type = DecodeTimezoneAbbrev(0, lowzone, &abbrev_offset, &tzp);
+		if (type == TZ || type == DTZ)
+			noop = (abbrev_offset == 0);
+		else if (type == DYNTZ)
+		{
+			/*
+			 * An abbreviation of a single-offset timezone ought not to be
+			 * configured as a DYNTZ, so don't bother checking.
+			 */
+		}
+		else
+		{
+			long		tzname_offset;
+
+			tzp = pg_tzset(tzname);
+			if (tzp && pg_get_timezone_offset(tzp, &tzname_offset))
+				noop = (tzname_offset == 0);
+		}
+
+		if (noop)
+		{
+			Node	   *timestamp = (Node *) lsecond(expr->args);
+
+			/* Strip any existing RelabelType node(s) */
+			while (timestamp && IsA(timestamp, RelabelType))
+				timestamp = (Node *) ((RelabelType *) timestamp)->arg;
+
+			/*
+			 * Replace the FuncExpr with its timestamp argument, relabeled as
+			 * though the function call had computed it.
+			 */
+			ret = (Node *) makeRelabelType((Expr *) timestamp,
+										   exprType(func_node),
+										   exprTypmod(func_node),
+										   exprCollation(func_node),
+										   COERCE_EXPLICIT_CAST);
+		}
+	}
+
+	PG_RETURN_POINTER(ret);
+}
+
 /*	timestamp_zone()
  *	Encode timestamp type with specified time zone.
  *	This function is just timestamp2timestamptz() except instead of
@@ -4963,6 +5045,52 @@ timestamp_zone(PG_FUNCTION_ARGS)
 	PG_RETURN_TIMESTAMPTZ(result);
 }
 
+/* timestamp_izone_transform()
+ * If we deduce at plan time that a particular timestamp_izone() or
+ * timestamptz_izone() call can only compute tz=0, the call will always return
+ * its second argument unchanged.  Simplify the expression tree accordingly.
+ */
+Datum
+timestamp_izone_transform(PG_FUNCTION_ARGS)
+{
+	Node	   *func_node = (Node *) PG_GETARG_POINTER(0);
+	FuncExpr   *expr = (FuncExpr *) func_node;
+	Node	   *ret = NULL;
+	Node	   *zone_node;
+
+	Assert(IsA(expr, FuncExpr));
+	Assert(list_length(expr->args) == 2);
+
+	zone_node = (Node *) linitial(expr->args);
+
+	if (IsA(zone_node, Const) &&!((Const *) zone_node)->constisnull)
+	{
+		Interval   *zone;
+
+		zone = DatumGetIntervalP(((Const *) zone_node)->constvalue);
+		if (zone->month == 0 && zone->day == 0 && zone->time == 0)
+		{
+			Node	   *timestamp = (Node *) lsecond(expr->args);
+
+			/* Strip any existing RelabelType node(s) */
+			while (timestamp && IsA(timestamp, RelabelType))
+				timestamp = (Node *) ((RelabelType *) timestamp)->arg;
+
+			/*
+			 * Replace the FuncExpr with its timestamp argument, relabeled as
+			 * though the function call had computed it.
+			 */
+			ret = (Node *) makeRelabelType((Expr *) timestamp,
+										   exprType(func_node),
+										   exprTypmod(func_node),
+										   exprCollation(func_node),
+										   COERCE_EXPLICIT_CAST);
+		}
+	}
+
+	PG_RETURN_POINTER(ret);
+}
+
 /* timestamp_izone()
  * Encode timestamp type with specified time interval as time zone.
  */
diff --git a/src/include/catalog/pg_proc.h b/src/include/catalog/pg_proc.h
index 9edfdb8..0a31ac1 100644
--- a/src/include/catalog/pg_proc.h
+++ b/src/include/catalog/pg_proc.h
@@ -1155,7 +1155,9 @@ DATA(insert OID = 999 (  lseg_eq		   PGNSP PGUID 12 1 0 0 0 f f f t t f i 2 0 16
 
 /* OIDS 1000 - 1999 */
 
-DATA(insert OID = 1026 (  timezone		   PGNSP PGUID 12 1 0 0 0 f f f f t f i 2 0 1114 "1186 1184" _null_ _null_ _null_ _null_ timestamptz_izone _null_ _null_ _null_ ));
+DATA(insert OID = 3994 (  timestamp_izone_transform PGNSP PGUID 12 1 0 0 0 f f f f t f i 1 0 2281 "2281" _null_ _null_ _null_ _null_ timestamp_izone_transform _null_ _null_ _null_ ));
+DESCR("transform a time zone adjustment");
+DATA(insert OID = 1026 (  timezone		   PGNSP PGUID 12 1 0 0 timestamp_izone_transform f f f f t f i 2 0 1114 "1186 1184" _null_ _null_ _null_ _null_ timestamptz_izone _null_ _null_ _null_ ));
 DESCR("adjust timestamp to new time zone");
 
 DATA(insert OID = 1031 (  aclitemin		   PGNSP PGUID 12 1 0 0 0 f f f f t f s 1 0 1033 "2275" _null_ _null_ _null_ _null_ aclitemin _null_ _null_ _null_ ));
@@ -1269,7 +1271,9 @@ DATA(insert OID = 1156 (  timestamptz_ge   PGNSP PGUID 12 1 0 0 0 f f f t t f i
 DATA(insert OID = 1157 (  timestamptz_gt   PGNSP PGUID 12 1 0 0 0 f f f t t f i 2 0 16 "1184 1184" _null_ _null_ _null_ _null_ timestamp_gt _null_ _null_ _null_ ));
 DATA(insert OID = 1158 (  to_timestamp	   PGNSP PGUID 14 1 0 0 0 f f f f t f i 1 0 1184 "701" _null_ _null_ _null_ _null_ "select (''epoch''::pg_catalog.timestamptz + $1 * ''1 second''::pg_catalog.interval)" _null_ _null_ _null_ ));
 DESCR("convert UNIX epoch to timestamptz");
-DATA(insert OID = 1159 (  timezone		   PGNSP PGUID 12 1 0 0 0 f f f f t f i 2 0 1114 "25 1184" _null_ _null_ _null_ _null_	timestamptz_zone _null_ _null_ _null_ ));
+DATA(insert OID = 3995 (  timestamp_zone_transform PGNSP PGUID 12 1 0 0 0 f f f f t f i 1 0 2281 "2281" _null_ _null_ _null_ _null_ timestamp_zone_transform _null_ _null_ _null_ ));
+DESCR("transform a time zone adjustment");
+DATA(insert OID = 1159 (  timezone		   PGNSP PGUID 12 1 0 0 timestamp_zone_transform f f f f t f i 2 0 1114 "25 1184" _null_ _null_ _null_ _null_	timestamptz_zone _null_ _null_ _null_ ));
 DESCR("adjust timestamp to new time zone");
 
 DATA(insert OID = 1160 (  interval_in	   PGNSP PGUID 12 1 0 0 0 f f f f t f s 3 0 1186 "2275 26 23" _null_ _null_ _null_ _null_ interval_in _null_ _null_ _null_ ));
@@ -2994,9 +2998,9 @@ DESCR("date difference preserving months and years");
 DATA(insert OID = 2059 (  age				PGNSP PGUID 14 1 0 0 0 f f f f t f s 1 0 1186 "1114" _null_ _null_ _null_ _null_ "select pg_catalog.age(cast(current_date as timestamp without time zone), $1)" _null_ _null_ _null_ ));
 DESCR("date difference from today preserving months and years");
 
-DATA(insert OID = 2069 (  timezone			PGNSP PGUID 12 1 0 0 0 f f f f t f i 2 0 1184 "25 1114" _null_ _null_ _null_ _null_ timestamp_zone _null_ _null_ _null_ ));
+DATA(insert OID = 2069 (  timezone			PGNSP PGUID 12 1 0 0 timestamp_zone_transform f f f f t f i 2 0 1184 "25 1114" _null_ _null_ _null_ _null_ timestamp_zone _null_ _null_ _null_ ));
 DESCR("adjust timestamp to new time zone");
-DATA(insert OID = 2070 (  timezone			PGNSP PGUID 12 1 0 0 0 f f f f t f i 2 0 1184 "1186 1114" _null_ _null_ _null_ _null_	timestamp_izone _null_ _null_ _null_ ));
+DATA(insert OID = 2070 (  timezone			PGNSP PGUID 12 1 0 0 timestamp_izone_transform f f f f t f i 2 0 1184 "1186 1114" _null_ _null_ _null_ _null_	timestamp_izone _null_ _null_ _null_ ));
 DESCR("adjust timestamp to new time zone");
 DATA(insert OID = 2071 (  date_pl_interval	PGNSP PGUID 12 1 0 0 0 f f f f t f i 2 0 1114 "1082 1186" _null_ _null_ _null_ _null_	date_pl_interval _null_ _null_ _null_ ));
 DATA(insert OID = 2072 (  date_mi_interval	PGNSP PGUID 12 1 0 0 0 f f f f t f i 2 0 1114 "1082 1186" _null_ _null_ _null_ _null_	date_mi_interval _null_ _null_ _null_ ));
diff --git a/src/include/utils/timestamp.h b/src/include/utils/timestamp.h
index 70118f5..530fef1 100644
--- a/src/include/utils/timestamp.h
+++ b/src/include/utils/timestamp.h
@@ -161,7 +161,9 @@ extern Datum timestamp_trunc(PG_FUNCTION_ARGS);
 extern Datum interval_trunc(PG_FUNCTION_ARGS);
 extern Datum timestamp_part(PG_FUNCTION_ARGS);
 extern Datum interval_part(PG_FUNCTION_ARGS);
+extern Datum timestamp_zone_transform(PG_FUNCTION_ARGS);
 extern Datum timestamp_zone(PG_FUNCTION_ARGS);
+extern Datum timestamp_izone_transform(PG_FUNCTION_ARGS);
 extern Datum timestamp_izone(PG_FUNCTION_ARGS);
 extern Datum timestamp_timestamptz(PG_FUNCTION_ARGS);