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Date: Sun, 2 Sep 2018 22:11:59 -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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fOn Sun, Sep  2, 2018 at 02:21:58PM -0400, Tom Lane wrote:
> Bruce Momjian <bruce@momjian.us> writes:
> > !     The <literal>AT TIME ZONE</literal> construct allows the addition,
> > !     conversion, and removal of time zones for time stamp values.  <xref
> >       linkend="functions-datetime-zoneconvert-table"/> shows its
> >       variants.
> 
> Maybe it'd be more to the point to say that it allows conversion between
> "timestamp with time zone" and "timestamp without time zone".
> 
> > !     The first example takes a time stamp without time zone and interprets
> > !     it in the MST time zone (UTC-7), returning a time stamp with time
> > !     zone value which is displayed in local time (PST, UTC-8).  The second
> > !     example takes a time stamp with time zone value (EST, UTC-5) and
> > !     converts it to the date and time in MST (UTC-7) without time zone.
> > !     Basically, the first example takes the date and time and puts it in
> > !     the specified time zone.  The second example takes a time stamp with
> > !     time zone and shifts it to the specified time zone. (No time zone
> > !     designation is returned.)
> 
> 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.

Is there any value to showing these two queries which show how calling
AT TIME ZONE twice cancels itself out:

	SELECT '2018-09-02 07:09:19'::timestamp AT TIME ZONE 'America/Chicago' AT TIME ZONE 'America/Chicago';
	      timezone
	---------------------
	 2018-09-02 07:09:19
	
	SELECT '2018-09-02 07:09:19-04'::timestamptz AT TIME ZONE 'America/Chicago' AT TIME ZONE 'America/Chicago';
	        timezone
	------------------------
	 2018-09-02 07:09:19-04

or this one which shows how to convert a date/time from one time zone to
another:

	SELECT '2018-09-02 07:09:19'::timestamp AT TIME ZONE 'Asia/Tokyo' AT
	TIME ZONE 'America/Chicago';
	      timezone
	---------------------
	 2018-09-01 17:09:19

-- 
  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
new file mode 100644
index bb794e0..5e3d54f
*** a/doc/src/sgml/func.sgml
--- b/doc/src/sgml/func.sgml
*************** SELECT date_trunc('year', TIMESTAMP '200
*** 8082,8091 ****
     </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.
     </para>
  
      <table id="functions-datetime-zoneconvert-table">
--- 8082,8092 ----
     </indexterm>
  
     <para>
!     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">
*************** SELECT TIMESTAMP '2001-02-16 20:38:40' A
*** 8145,8153 ****
  SELECT TIMESTAMP WITH TIME ZONE '2001-02-16 20:38:40-05' AT TIME ZONE 'MST';
  <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).
     </para>
  
     <para>
--- 8146,8158 ----
  SELECT TIMESTAMP WITH TIME ZONE '2001-02-16 20:38:40-05' AT TIME ZONE 'MST';
  <lineannotation>Result: </lineannotation><computeroutput>2001-02-16 18:38:40</computeroutput>
  </screen>
!     The first example adds a time zone to a value that lacks it.
!     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> value.  Converting
!     <emphasis>time</emphasis> values to other time zones uses the currently
!     active time zone rules since no date is present.
     </para>
  
     <para>

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