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Date: Mon, 3 Sep 2018 09:20:34 -0400
From: Bruce Momjian <bruce@momjian.us>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: PostgreSQL-documentation <pgsql-docs@postgresql.org>
Subject: Re: AT TIME ZONE correction
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On Sun, Sep  2, 2018 at 10:11:59PM -0400, Bruce Momjian wrote:
> On Sun, Sep  2, 2018 at 02:21:58PM -0400, Tom Lane wrote:
> > I still find this to be more confusing than helpful.  In particular,
> > I do not think that it's possible to explain this behavior clearly
> > without mentioning that timestamp with time zone values are always
> > stored in UTC and what AT TIME ZONE really does is convert between UTC
> > and the specified zone (in a direction dependent on which type is
> > supplied as argument).
> 
> Agreed.  The more I dig into this the more I learn.  I have developed
> the attached patch which I hope this time is an improvement.

I polished the text some more and changed the three-letter time zone
abbreviation (e.g., MST) to use the more general text, e.g.
"America/Denver".  We should not be encouraging people to specify the
daylight savings time status based on the date in the date/time string.

-- 
  Bruce Momjian  <bruce@momjian.us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

+ As you are, so once was I.  As I am, so you will be. +
+                      Ancient Roman grave inscription +

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diff --git a/doc/src/sgml/func.sgml b/doc/src/sgml/func.sgml
index bb794e0..2135799 100644
--- a/doc/src/sgml/func.sgml
+++ b/doc/src/sgml/func.sgml
@@ -8082,10 +8082,11 @@ SELECT date_trunc('year', TIMESTAMP '2001-02-16 20:38:40');
    </indexterm>
 
    <para>
-    The <literal>AT TIME ZONE</literal> construct allows conversions
-    of time stamps to different time zones.  <xref
-    linkend="functions-datetime-zoneconvert-table"/> shows its
-    variants.
+    The <literal>AT TIME ZONE</literal> converts time
+    stamp <emphasis>without time zone</emphasis> to/from
+    time stamp <emphasis>with time zone</emphasis>, and
+    <emphasis>time</emphasis> values to different time zones. <xref
+    linkend="functions-datetime-zoneconvert-table"/> shows its variants.
    </para>
 
     <table id="functions-datetime-zoneconvert-table">
@@ -8130,24 +8131,29 @@ SELECT date_trunc('year', TIMESTAMP '2001-02-16 20:38:40');
 
    <para>
     In these expressions, the desired time zone <replaceable>zone</replaceable> can be
-    specified either as a text string (e.g., <literal>'PST'</literal>)
+    specified either as a text string (e.g., <literal>'America/Los_Angeles'</literal>)
     or as an interval (e.g., <literal>INTERVAL '-08:00'</literal>).
     In the text case, a time zone name can be specified in any of the ways
     described in <xref linkend="datatype-timezones"/>.
    </para>
 
    <para>
-    Examples (assuming the local time zone is <literal>PST8PDT</literal>):
+    Examples (assuming the local time zone is <literal>America/Los_Angeles</literal>):
 <screen>
-SELECT TIMESTAMP '2001-02-16 20:38:40' AT TIME ZONE 'MST';
+SELECT TIMESTAMP '2001-02-16 20:38:40' AT TIME ZONE 'America/Denver';
 <lineannotation>Result: </lineannotation><computeroutput>2001-02-16 19:38:40-08</computeroutput>
 
-SELECT TIMESTAMP WITH TIME ZONE '2001-02-16 20:38:40-05' AT TIME ZONE 'MST';
+SELECT TIMESTAMP WITH TIME ZONE '2001-02-16 20:38:40-05' AT TIME ZONE 'America/Denver';
 <lineannotation>Result: </lineannotation><computeroutput>2001-02-16 18:38:40</computeroutput>
 </screen>
-    The first example takes a time stamp without time zone and interprets it as MST time
-    (UTC-7), which is then converted to PST (UTC-8) for display.  The second example takes
-    a time stamp specified in EST (UTC-5) and converts it to local time in MST (UTC-7).
+    The first example adds a time zone to a value that lacks it, and
+    displays the value using the current <varname>TimeZone</varname>
+    setting.  The second example shifts the time stamp with time zone
+    value to the specified time zone, and returns the value without a
+    time zone.  This allows storage and display of values different
+    from the current <varname>TimeZone</varname> setting.  Converting
+    <emphasis>time</emphasis> values to other time zones uses the currently
+    active time zone rules since no date is supplied.
    </para>
 
    <para>

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