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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.5. POSIX Time Zone Specifications</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-config-files.html" title="B.4. Date/Time Configuration Files" /><link rel="next" href="datetime-units-history.html" title="B.6. History of Units" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">B.5. <acronym class="acronym">POSIX</acronym> Time Zone Specifications</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="datetime-config-files.html" title="B.4. Date/Time Configuration Files">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 16.3 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="datetime-units-history.html" title="B.6. History of Units">Next</a></td></tr></table><hr /></div><div class="sect1" id="DATETIME-POSIX-TIMEZONE-SPECS"><div class="titlepage"><div><div><h2 class="title" style="clear: both">B.5. <acronym class="acronym">POSIX</acronym> Time Zone Specifications <a href="#DATETIME-POSIX-TIMEZONE-SPECS" class="id_link">#</a></h2></div></div></div><a id="id-1.11.3.8.2" class="indexterm"></a><p>
<span class="productname">PostgreSQL</span> can accept time zone specifications
that are written according to the <acronym class="acronym">POSIX</acronym> standard's rules
for the <code class="varname">TZ</code> environment
variable. <acronym class="acronym">POSIX</acronym> time zone specifications are
inadequate to deal with the complexity of real-world time zone history,
but there are sometimes reasons to use them.
</p><p>
A POSIX time zone specification has the form
</p><pre class="synopsis">
<em class="replaceable"><code>STD</code></em> <em class="replaceable"><code>offset</code></em> [<span class="optional"> <em class="replaceable"><code>DST</code></em> [<span class="optional"> <em class="replaceable"><code>dstoffset</code></em> </span>] [<span class="optional"> , <em class="replaceable"><code>rule</code></em> </span>] </span>]
</pre><p>
(For readability, we show spaces between the fields, but spaces should
not be used in practice.) The fields are:
</p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p>
<em class="replaceable"><code>STD</code></em> is the zone abbreviation to be used
for standard time.
</p></li><li class="listitem"><p>
<em class="replaceable"><code>offset</code></em> is the zone's standard-time offset
from UTC.
</p></li><li class="listitem"><p>
<em class="replaceable"><code>DST</code></em> is the zone abbreviation to be used
for daylight-savings time. If this field and the following ones are
omitted, the zone uses a fixed UTC offset with no daylight-savings
rule.
</p></li><li class="listitem"><p>
<em class="replaceable"><code>dstoffset</code></em> is the daylight-savings offset
from UTC. This field is typically omitted, since it defaults to one
hour less than the standard-time <em class="replaceable"><code>offset</code></em>,
which is usually the right thing.
</p></li><li class="listitem"><p>
<em class="replaceable"><code>rule</code></em> defines the rule for when daylight
savings is in effect, as described below.
</p></li></ul></div><p>
</p><p>
In this syntax, a zone abbreviation can be a string of letters, such
as <code class="literal">EST</code>, or an arbitrary string surrounded by angle
brackets, such as <code class="literal"><UTC-05></code>.
Note that the zone abbreviations given here are only used for output,
and even then only in some timestamp output formats. The zone
abbreviations recognized in timestamp input are determined as explained
in <a class="xref" href="datetime-config-files.html" title="B.4. Date/Time Configuration Files">Section B.4</a>.
</p><p>
The offset fields specify the hours, and optionally minutes and seconds,
difference from UTC. They have the format
<em class="replaceable"><code>hh</code></em>[<span class="optional"><code class="literal">:</code><em class="replaceable"><code>mm</code></em>[<span class="optional"><code class="literal">:</code><em class="replaceable"><code>ss</code></em></span>]</span>]
optionally with a leading sign (<code class="literal">+</code>
or <code class="literal">-</code>). The positive sign is used for
zones <span class="emphasis"><em>west</em></span> of Greenwich. (Note that this is the
opposite of the ISO-8601 sign convention used elsewhere in
<span class="productname">PostgreSQL</span>.) <em class="replaceable"><code>hh</code></em>
can have one or two digits; <em class="replaceable"><code>mm</code></em>
and <em class="replaceable"><code>ss</code></em> (if used) must have two.
</p><p>
The daylight-savings transition <em class="replaceable"><code>rule</code></em> has the
format
</p><pre class="synopsis">
<em class="replaceable"><code>dstdate</code></em> [<span class="optional"> <code class="literal">/</code> <em class="replaceable"><code>dsttime</code></em> </span>] <code class="literal">,</code> <em class="replaceable"><code>stddate</code></em> [<span class="optional"> <code class="literal">/</code> <em class="replaceable"><code>stdtime</code></em> </span>]
</pre><p>
(As before, spaces should not be included in practice.)
The <em class="replaceable"><code>dstdate</code></em>
and <em class="replaceable"><code>dsttime</code></em> fields define when daylight-savings
time starts, while <em class="replaceable"><code>stddate</code></em>
and <em class="replaceable"><code>stdtime</code></em> define when standard time
starts. (In some cases, notably in zones south of the equator, the
former might be later in the year than the latter.) The date fields
have one of these formats:
</p><div class="variablelist"><dl class="variablelist"><dt><span class="term"><em class="replaceable"><code>n</code></em></span></dt><dd><p>
A plain integer denotes a day of the year, counting from zero to
364, or to 365 in leap years.
</p></dd><dt><span class="term"><code class="literal">J</code><em class="replaceable"><code>n</code></em></span></dt><dd><p>
In this form, <em class="replaceable"><code>n</code></em> counts from 1 to 365,
and February 29 is not counted even if it is present. (Thus, a
transition occurring on February 29 could not be specified this
way. However, days after February have the same numbers whether
it's a leap year or not, so that this form is usually more useful
than the plain-integer form for transitions on fixed dates.)
</p></dd><dt><span class="term"><code class="literal">M</code><em class="replaceable"><code>m</code></em><code class="literal">.</code><em class="replaceable"><code>n</code></em><code class="literal">.</code><em class="replaceable"><code>d</code></em></span></dt><dd><p>
This form specifies a transition that always happens during the same
month and on the same day of the week. <em class="replaceable"><code>m</code></em>
identifies the month, from 1 to 12. <em class="replaceable"><code>n</code></em>
specifies the <em class="replaceable"><code>n</code></em>'th occurrence of the
weekday identified by <em class="replaceable"><code>d</code></em>.
<em class="replaceable"><code>n</code></em> is a number between 1 and 4, or 5
meaning the last occurrence of that weekday in the month (which
could be the fourth or the fifth). <em class="replaceable"><code>d</code></em> is
a number between 0 and 6, with 0 indicating Sunday.
For example, <code class="literal">M3.2.0</code> means <span class="quote">“<span class="quote">the second
Sunday in March</span>”</span>.
</p></dd></dl></div><p>
</p><div class="note"><h3 class="title">Note</h3><p>
The <code class="literal">M</code> format is sufficient to describe many common
daylight-savings transition laws. But note that none of these variants
can deal with daylight-savings law changes, so in practice the
historical data stored for named time zones (in the IANA time zone
database) is necessary to interpret past time stamps correctly.
</p></div><p>
The time fields in a transition rule have the same format as the offset
fields described previously, except that they cannot contain signs.
They define the current local time at which the change to the other
time occurs. If omitted, they default to <code class="literal">02:00:00</code>.
</p><p>
If a daylight-savings abbreviation is given but the
transition <em class="replaceable"><code>rule</code></em> field is omitted,
the fallback behavior is to use the
rule <code class="literal">M3.2.0,M11.1.0</code>, which corresponds to USA
practice as of 2020 (that is, spring forward on the second Sunday of
March, fall back on the first Sunday of November, both transitions
occurring at 2AM prevailing time). Note that this rule does not
give correct USA transition dates for years before 2007.
</p><p>
As an example, <code class="literal">CET-1CEST,M3.5.0,M10.5.0/3</code> describes
current (as of 2020) timekeeping practice in Paris. This specification
says that standard time has the abbreviation <code class="literal">CET</code> and
is one hour ahead (east) of UTC; daylight savings time has the
abbreviation <code class="literal">CEST</code> and is implicitly two hours ahead
of UTC; daylight savings time begins on the last Sunday in March at 2AM
CET and ends on the last Sunday in October at 3AM CEST.
</p><p>
The four timezone names <code class="literal">EST5EDT</code>,
<code class="literal">CST6CDT</code>, <code class="literal">MST7MDT</code>,
and <code class="literal">PST8PDT</code> look like they are POSIX zone
specifications. However, they actually are treated as named time zones
because (for historical reasons) there are files by those names in the
IANA time zone database. The practical implication of this is that
these zone names will produce valid historical USA daylight-savings
transitions, even when a plain POSIX specification would not.
</p><p>
One should be wary that it is easy to misspell a POSIX-style time zone
specification, since there is no check on the reasonableness of the
zone abbreviation(s). For example, <code class="literal">SET TIMEZONE TO
FOOBAR0</code> will work, leaving the system effectively using a
rather peculiar abbreviation for UTC.
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