diff --git a/doc/src/sgml/datetime.sgml b/doc/src/sgml/datetime.sgml
index 7da4d0b7789..71fbf842cca 100644
--- a/doc/src/sgml/datetime.sgml
+++ b/doc/src/sgml/datetime.sgml
@@ -757,6 +757,18 @@
    CET and ends on the last Sunday in October at 3AM CEST.
   </para>
 
+  <para>
+   The four timezone names <literal>EST5EDT</literal>,
+   <literal>CST6CDT</literal>, <literal>MST7MDT</literal>,
+   and <literal>PST8PDT</literal> look like they are POSIX zone
+   specifications.  However, they actually are treated as named time zones
+   because (for historical reasons) there are files by those names in the
+   IANA time zone database.  The practical implication of this is that
+   these zone names will produce valid historical USA daylight-savings
+   transitions, even when a plain POSIX specification would not due to
+   lack of a suitable <filename>posixrules</filename> file.
+  </para>
+
   <para>
    One should be wary that it is easy to misspell a POSIX-style time zone
    specification, since there is no check on the reasonableness of the
diff --git a/doc/src/sgml/func.sgml b/doc/src/sgml/func.sgml
index 9d71678029e..b7c450ea29d 100644
--- a/doc/src/sgml/func.sgml
+++ b/doc/src/sgml/func.sgml
@@ -9131,18 +9131,22 @@ SELECT (DATE '2001-10-30', DATE '2001-10-30') OVERLAPS
    When adding an <type>interval</type> value to (or subtracting an
    <type>interval</type> value from) a <type>timestamp with time zone</type>
    value, the days component advances or decrements the date of the
-   <type>timestamp with time zone</type> by the indicated number of days.
+   <type>timestamp with time zone</type> by the indicated number of days,
+   keeping the time of day the same.
    Across daylight saving time changes (when the session time zone is set to a
    time zone that recognizes DST), this means <literal>interval '1 day'</literal>
    does not necessarily equal <literal>interval '24 hours'</literal>.
-   For example, with the session time zone set to <literal>CST7CDT</literal>,
-   <literal>timestamp with time zone '2005-04-02 12:00-07' + interval '1 day'</literal>
-   will produce <literal>timestamp with time zone '2005-04-03 12:00-06'</literal>,
-   while adding <literal>interval '24 hours'</literal> to the same initial
-   <type>timestamp with time zone</type> produces
-   <literal>timestamp with time zone '2005-04-03 13:00-06'</literal>, as there is
-   a change in daylight saving time at <literal>2005-04-03 02:00</literal> in time zone
-   <literal>CST7CDT</literal>.
+   For example, with the session time zone set
+   to <literal>America/Denver</literal>:
+<screen>
+SELECT timestamp with time zone '2005-04-02 12:00:00-07' + interval '1 day';
+<lineannotation>Result: </lineannotation><computeroutput>2005-04-03 12:00:00-06</computeroutput>
+SELECT timestamp with time zone '2005-04-02 12:00:00-07' + interval '24 hours';
+<lineannotation>Result: </lineannotation><computeroutput>2005-04-03 13:00:00-06</computeroutput>
+</screen>
+   This happens because an hour was skipped due to a change in daylight saving
+   time at <literal>2005-04-03 02:00:00</literal> in time zone
+   <literal>America/Denver</literal>.
   </para>
 
   <para>
diff --git a/src/test/regress/expected/horology.out b/src/test/regress/expected/horology.out
index f67d624ad5d..c8c33a0fc06 100644
--- a/src/test/regress/expected/horology.out
+++ b/src/test/regress/expected/horology.out
@@ -652,7 +652,8 @@ SELECT (timestamp with time zone 'tomorrow' > 'now') as "True";
 (1 row)
 
 -- timestamp with time zone, interval arithmetic around DST change
-SET TIME ZONE 'CST7CDT';
+-- (just for fun, let's use an intentionally nonstandard POSIX zone spec)
+SET TIME ZONE 'CST7CDT,M4.1.0,M10.5.0';
 SELECT timestamp with time zone '2005-04-02 12:00-07' + interval '1 day' as "Apr 3, 12:00";
          Apr 3, 12:00         
 ------------------------------
diff --git a/src/test/regress/sql/horology.sql b/src/test/regress/sql/horology.sql
index d1f6d5bfcd3..c464e6766c6 100644
--- a/src/test/regress/sql/horology.sql
+++ b/src/test/regress/sql/horology.sql
@@ -122,7 +122,8 @@ SELECT (timestamp with time zone 'tomorrow' = (timestamp with time zone 'yesterd
 SELECT (timestamp with time zone 'tomorrow' > 'now') as "True";
 
 -- timestamp with time zone, interval arithmetic around DST change
-SET TIME ZONE 'CST7CDT';
+-- (just for fun, let's use an intentionally nonstandard POSIX zone spec)
+SET TIME ZONE 'CST7CDT,M4.1.0,M10.5.0';
 SELECT timestamp with time zone '2005-04-02 12:00-07' + interval '1 day' as "Apr 3, 12:00";
 SELECT timestamp with time zone '2005-04-02 12:00-07' + interval '24 hours' as "Apr 3, 13:00";
 SELECT timestamp with time zone '2005-04-03 12:00-06' - interval '1 day' as "Apr 2, 12:00";