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 <[email protected]> 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 +
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>
