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><H1
CLASS="SECT1"
><A
NAME="DATETIME-POSIX-TIMEZONE-SPECS"
>B.5. <ACRONYM
CLASS="ACRONYM"
>POSIX</ACRONYM
> Time Zone Specifications</A
></H1
><P
>   <ACRONYM
CLASS="ACRONYM"
>PostgreSQL</ACRONYM
> can accept time zone specifications that
   are written according to the <ACRONYM
CLASS="ACRONYM"
>POSIX</ACRONYM
> standard's rules
   for the <TT
CLASS="VARNAME"
>TZ</TT
> 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"
><TT
CLASS="REPLACEABLE"
><I
>STD</I
></TT
> <TT
CLASS="REPLACEABLE"
><I
>offset</I
></TT
> [<SPAN
CLASS="OPTIONAL"
> <TT
CLASS="REPLACEABLE"
><I
>DST</I
></TT
> [<SPAN
CLASS="OPTIONAL"
> <TT
CLASS="REPLACEABLE"
><I
>dstoffset</I
></TT
> </SPAN
>] [<SPAN
CLASS="OPTIONAL"
> , <TT
CLASS="REPLACEABLE"
><I
>rule</I
></TT
> </SPAN
>] </SPAN
>]</PRE
><P>
   (For readability, we show spaces between the fields, but spaces should
   not be used in practice.)  The fields are:
   <P
></P
></P><UL
><LI
><P
>      <TT
CLASS="REPLACEABLE"
><I
>STD</I
></TT
> is the zone abbreviation to be used
      for standard time.
     </P
></LI
><LI
><P
>      <TT
CLASS="REPLACEABLE"
><I
>offset</I
></TT
> is the zone's standard-time offset
      from UTC.
     </P
></LI
><LI
><P
>      <TT
CLASS="REPLACEABLE"
><I
>DST</I
></TT
> 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
><P
>      <TT
CLASS="REPLACEABLE"
><I
>dstoffset</I
></TT
> is the daylight-savings offset
      from UTC.  This field is typically omitted, since it defaults to one
      hour less than the standard-time <TT
CLASS="REPLACEABLE"
><I
>offset</I
></TT
>,
      which is usually the right thing.
     </P
></LI
><LI
><P
>      <TT
CLASS="REPLACEABLE"
><I
>rule</I
></TT
> defines the rule for when daylight
      savings is in effect, as described below.
     </P
></LI
></UL
><P>
  </P
><P
>   In this syntax, a zone abbreviation can be a string of letters, such
   as <TT
CLASS="LITERAL"
>EST</TT
>, or an arbitrary string surrounded by angle
   brackets, such as <TT
CLASS="LITERAL"
>&lt;UTC-05&gt;</TT
>.
   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
HREF="datetime-config-files.html"
>Section B.4</A
>.
  </P
><P
>   The offset fields specify the hours, and optionally minutes and seconds,
   difference from UTC.  They have the format
   <TT
CLASS="REPLACEABLE"
><I
>hh</I
></TT
>[<SPAN
CLASS="OPTIONAL"
><TT
CLASS="LITERAL"
>:</TT
><TT
CLASS="REPLACEABLE"
><I
>mm</I
></TT
>[<SPAN
CLASS="OPTIONAL"
><TT
CLASS="LITERAL"
>:</TT
><TT
CLASS="REPLACEABLE"
><I
>ss</I
></TT
></SPAN
>]</SPAN
>]
   optionally with a leading sign (<TT
CLASS="LITERAL"
>+</TT
>
   or <TT
CLASS="LITERAL"
>-</TT
>).  The positive sign is used for
   zones <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>west</I
></SPAN
> of Greenwich.  (Note that this is the
   opposite of the ISO-8601 sign convention used elsewhere in
   <ACRONYM
CLASS="ACRONYM"
>PostgreSQL</ACRONYM
>.)  <TT
CLASS="REPLACEABLE"
><I
>hh</I
></TT
> can have
   one or two digits; <TT
CLASS="REPLACEABLE"
><I
>mm</I
></TT
>
   and <TT
CLASS="REPLACEABLE"
><I
>ss</I
></TT
> (if used) must have two.
  </P
><P
>   The daylight-savings transition <TT
CLASS="REPLACEABLE"
><I
>rule</I
></TT
> has the
   format
</P><PRE
CLASS="SYNOPSIS"
><TT
CLASS="REPLACEABLE"
><I
>dstdate</I
></TT
> [<SPAN
CLASS="OPTIONAL"
> <TT
CLASS="LITERAL"
>/</TT
> <TT
CLASS="REPLACEABLE"
><I
>dsttime</I
></TT
> </SPAN
>] <TT
CLASS="LITERAL"
>,</TT
> <TT
CLASS="REPLACEABLE"
><I
>stddate</I
></TT
> [<SPAN
CLASS="OPTIONAL"
> <TT
CLASS="LITERAL"
>/</TT
> <TT
CLASS="REPLACEABLE"
><I
>stdtime</I
></TT
> </SPAN
>]</PRE
><P>
   (As before, spaces should not be included in practice.)
   The <TT
CLASS="REPLACEABLE"
><I
>dstdate</I
></TT
>
   and <TT
CLASS="REPLACEABLE"
><I
>dsttime</I
></TT
> fields define when daylight-savings
   time starts, while <TT
CLASS="REPLACEABLE"
><I
>stddate</I
></TT
>
   and <TT
CLASS="REPLACEABLE"
><I
>stdtime</I
></TT
> 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
></P
></P><DIV
CLASS="VARIABLELIST"
><DL
><DT
><TT
CLASS="REPLACEABLE"
><I
>n</I
></TT
></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
><TT
CLASS="LITERAL"
>J</TT
><TT
CLASS="REPLACEABLE"
><I
>n</I
></TT
></DT
><DD
><P
>       In this form, <TT
CLASS="REPLACEABLE"
><I
>n</I
></TT
> 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
><TT
CLASS="LITERAL"
>M</TT
><TT
CLASS="REPLACEABLE"
><I
>m</I
></TT
><TT
CLASS="LITERAL"
>.</TT
><TT
CLASS="REPLACEABLE"
><I
>n</I
></TT
><TT
CLASS="LITERAL"
>.</TT
><TT
CLASS="REPLACEABLE"
><I
>d</I
></TT
></DT
><DD
><P
>       This form specifies a transition that always happens during the same
       month and on the same day of the week.  <TT
CLASS="REPLACEABLE"
><I
>m</I
></TT
>
       identifies the month, from 1 to 12.  <TT
CLASS="REPLACEABLE"
><I
>n</I
></TT
>
       specifies the <TT
CLASS="REPLACEABLE"
><I
>n</I
></TT
>'th occurrence of the
       weekday identified by <TT
CLASS="REPLACEABLE"
><I
>d</I
></TT
>.
       <TT
CLASS="REPLACEABLE"
><I
>n</I
></TT
> 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).  <TT
CLASS="REPLACEABLE"
><I
>d</I
></TT
> is
       a number between 0 and 6, with 0 indicating Sunday.
       For example, <TT
CLASS="LITERAL"
>M3.2.0</TT
> means <SPAN
CLASS="QUOTE"
>"the second
       Sunday in March"</SPAN
>.
      </P
></DD
></DL
></DIV
><P>
  </P
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
>    The <TT
CLASS="LITERAL"
>M</TT
> 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
></BLOCKQUOTE
></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 <TT
CLASS="LITERAL"
>02:00:00</TT
>.
  </P
><P
>   If a daylight-savings abbreviation is given but the
   transition <TT
CLASS="REPLACEABLE"
><I
>rule</I
></TT
> field is omitted,
   <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> attempts to determine the
   transition times by consulting the <TT
CLASS="FILENAME"
>posixrules</TT
> file
   in the IANA time zone database.  This file has the same format as a
   full time zone entry, but only its transition timing rules are used,
   not its UTC offsets.  Typically, this file has the same contents as the
   <TT
CLASS="LITERAL"
>US/Eastern</TT
> file, so that POSIX-style time zone
   specifications follow USA daylight-savings rules.  If needed, you can
   adjust this behavior by replacing the <TT
CLASS="FILENAME"
>posixrules</TT
>
   file.
  </P
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
>    The facility to consult a <TT
CLASS="FILENAME"
>posixrules</TT
> file has
    been deprecated by IANA, and it is likely to go away in the future.
    One bug in this feature, which is unlikely to be fixed before it
    disappears, is that it fails to apply DST rules to dates after 2038.
   </P
></BLOCKQUOTE
></DIV
><P
>   If the <TT
CLASS="FILENAME"
>posixrules</TT
> file is not present,
   the fallback behavior is to use the
   rule <TT
CLASS="LITERAL"
>M3.2.0,M11.1.0</TT
>, 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).
  </P
><P
>   As an example, <TT
CLASS="LITERAL"
>CET-1CEST,M3.5.0,M10.5.0/3</TT
> describes
   current (as of 2020) timekeeping practice in Paris.  This specification
   says that standard time has the abbreviation <TT
CLASS="LITERAL"
>CET</TT
> and
   is one hour ahead (east) of UTC; daylight savings time has the
   abbreviation <TT
CLASS="LITERAL"
>CEST</TT
> 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 <TT
CLASS="LITERAL"
>EST5EDT</TT
>,
   <TT
CLASS="LITERAL"
>CST6CDT</TT
>, <TT
CLASS="LITERAL"
>MST7MDT</TT
>,
   and <TT
CLASS="LITERAL"
>PST8PDT</TT
> 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 due to
   lack of a suitable <TT
CLASS="FILENAME"
>posixrules</TT
> file.
  </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, <TT
CLASS="LITERAL"
>SET TIMEZONE TO
   FOOBAR0</TT
> will work, leaving the system effectively using a
   rather peculiar abbreviation for UTC.
  </P
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