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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>B.2. Handling of Invalid or Ambiguous Timestamps</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><link rel="prev" href="datetime-input-rules.html" title="B.1. Date/Time Input Interpretation" /><link rel="next" href="datetime-keywords.html" title="B.3. Date/Time Key Words" /></head><body><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">B.2. Handling of Invalid or Ambiguous Timestamps</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="datetime-input-rules.html" title="B.1. Date/Time Input Interpretation">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="datetime-appendix.html" title="Appendix B. Date/Time Support">Up</a></td><th width="60%" align="center">Appendix B. Date/Time Support</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 11.5 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="datetime-keywords.html" title="B.3. Date/Time Key Words">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="DATETIME-INVALID-INPUT"><div class="titlepage"><div><div><h2 class="title" style="clear: both">B.2. Handling of Invalid or Ambiguous Timestamps</h2></div></div></div><p>
Ordinarily, if a date/time string is syntactically valid but contains
out-of-range field values, an error will be thrown. For example, input
specifying the 31st of February will be rejected.
</p><p>
During a daylight-savings-time transition, it is possible for a
seemingly valid timestamp string to represent a nonexistent or ambiguous
timestamp. Such cases are not rejected; the ambiguity is resolved by
determining which UTC offset to apply. For example, supposing that the
<a class="xref" href="runtime-config-client.html#GUC-TIMEZONE">TimeZone</a> parameter is set
to <code class="literal">America/New_York</code>, consider
</p><pre class="programlisting">
=> SELECT '2018-03-11 02:30'::timestamptz;
timestamptz
------------------------
2018-03-11 03:30:00-04
(1 row)
</pre><p>
Because that day was a spring-forward transition date in that time zone,
there was no civil time instant 2:30AM; clocks jumped forward from 2AM
EST to 3AM EDT. <span class="productname">PostgreSQL</span> interprets the
given time as if it were standard time (UTC-5), which then renders as
3:30AM EDT (UTC-4).
</p><p>
Conversely, consider the behavior during a fall-back transition:
</p><pre class="programlisting">
=> SELECT '2018-11-04 02:30'::timestamptz;
timestamptz
------------------------
2018-11-04 02:30:00-05
(1 row)
</pre><p>
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, <span class="productname">PostgreSQL</span> interprets the given time
as if it were standard time (UTC-5). We can force the matter by
specifying daylight-savings time:
</p><pre class="programlisting">
=> SELECT '2018-11-04 02:30 EDT'::timestamptz;
timestamptz
------------------------
2018-11-04 01:30:00-05
(1 row)
</pre><p>
This timestamp could validly be rendered as either 2:30 UTC-4 or
1:30 UTC-5; the timestamp output code chooses the latter.
</p><p>
The precise rule that is applied in such cases is that an invalid
timestamp that appears to fall within a jump-forward daylight savings
transition is assigned the UTC offset that prevailed in the time zone
just before the transition, while an ambiguous timestamp that could fall
on either side of a jump-back transition is assigned the UTC offset that
prevailed just after the transition. In most time zones this is
equivalent to saying that <span class="quote">“<span class="quote">the standard-time interpretation is
preferred when in doubt</span>”</span>.
</p><p>
In all cases, the UTC offset associated with a timestamp can be
specified explicitly, using either a numeric UTC offset or a time zone
abbreviation that corresponds to a fixed UTC offset. The rule just
given applies only when it is necessary to infer a UTC offset for a time
zone in which the offset varies.
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