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Doc: fix bogus example about ambiguous timestamps.

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I had a brain fade in commit d32899157, and used 2:30AM as the
example timestamp for both spring-forward and fall-back cases.
But it's not actually ambiguous at all in the fall-back case,
because that transition is from 2AM to 1AM under USA rules.
Fix the example to use 1:30AM, which *is* ambiguous.

Noted while answering a question from Aleksander Alekseev.
Back-patch to all supported branches.

Discussion: https://postgr.es/m/2191355.1641828552@sss.pgh.pa.us
This commit is contained in:
Tom Lane 2022-01-10 11:46:16 -05:00
parent 1cd46f168f
commit 05cdda68f8
1 changed files with 14 additions and 16 deletions
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30
doc/src/sgml/datetime.sgml
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@ -210,27 +210,25 @@
<para> <para>
Conversely, consider the behavior during a fall-back transition: Conversely, consider the behavior during a fall-back transition:
<programlisting> <programlisting>
=&gt; SELECT '2018-11-04 02:30'::timestamptz; =&gt; SELECT '2018-11-04 01:30'::timestamptz;
timestamptz
------------------------
2018-11-04 02:30:00-05
(1 row)
</programlisting>
On that date, there were two possible interpretations of 2:30AM; there
was 2:30AM EDT, and then an hour later after the reversion to standard
time, there was 2:30AM EST.
Again, <productname>PostgreSQL</productname> interprets the given time
as if it were standard time (UTC-5). We can force the matter by
specifying daylight-savings time:
<programlisting>
=&gt; SELECT '2018-11-04 02:30 EDT'::timestamptz;
timestamptz timestamptz
------------------------ ------------------------
2018-11-04 01:30:00-05 2018-11-04 01:30:00-05
(1 row) (1 row)
</programlisting> </programlisting>
This timestamp could validly be rendered as either 2:30 UTC-4 or On that date, there were two possible interpretations of 1:30AM; there
1:30 UTC-5; the timestamp output code chooses the latter. was 1:30AM EDT, and then an hour later after clocks jumped back from
2AM EDT to 1AM EST, there was 1:30AM EST.
Again, <productname>PostgreSQL</productname> interprets the given time
as if it were standard time (UTC-5). We can force the other
interpretation by specifying daylight-savings time:
<programlisting>
=&gt; SELECT '2018-11-04 01:30 EDT'::timestamptz;
timestamptz
------------------------
2018-11-04 01:30:00-04
(1 row)
</programlisting>
</para> </para>
<para> <para>