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b2cbced9ee
Up to now, PG has assumed that any given timezone abbreviation (such as
"EDT") represents a constant GMT offset in the usage of any particular
region; we had a way to configure what that offset was, but not for it
to be changeable over time. But, as with most things horological, this
view of the world is too simplistic: there are numerous regions that have
at one time or another switched to a different GMT offset but kept using
the same timezone abbreviation. Almost the entire Russian Federation did
that a few years ago, and later this month they're going to do it again.
And there are similar examples all over the world.
To cope with this, invent the notion of a "dynamic timezone abbreviation",
which is one that is referenced to a particular underlying timezone
(as defined in the IANA timezone database) and means whatever it currently
means in that zone. For zones that use or have used daylight-savings time,
the standard and DST abbreviations continue to have the property that you
can specify standard or DST time and get that time offset whether or not
DST was theoretically in effect at the time. However, the abbreviations
mean what they meant at the time in question (or most recently before that
time) rather than being absolutely fixed.
The standard abbreviation-list files have been changed to use this behavior
for abbreviations that have actually varied in meaning since 1970. The
old simple-numeric definitions are kept for abbreviations that have not
changed, since they are a bit faster to resolve.
While this is clearly a new feature, it seems necessary to back-patch it
into all active branches, because otherwise use of Russian zone
abbreviations is going to become even more problematic than it already was.
This change supersedes the changes in commit 513d06ded
et al to modify the
fixed meanings of the Russian abbreviations; since we've not shipped that
yet, this will avoid an undesirably incompatible (not to mention incorrect)
change in behavior for timestamps between 2011 and 2014.
This patch makes some cosmetic changes in ecpglib to keep its usage of
datetime lookup tables as similar as possible to the backend code, but
doesn't do anything about the increasingly obsolete set of timezone
abbreviation definitions that are hard-wired into ecpglib. Whatever we
do about that will likely not be appropriate material for back-patching.
Also, a potential free() of a garbage pointer after an out-of-memory
failure in ecpglib has been fixed.
This patch also fixes pre-existing bugs in DetermineTimeZoneOffset() that
caused it to produce unexpected results near a timezone transition, if
both the "before" and "after" states are marked as standard time. We'd
only ever thought about or tested transitions between standard and DST
time, but that's not what's happening when a zone simply redefines their
base GMT offset.
In passing, update the SGML documentation to refer to the Olson/zoneinfo/
zic timezone database as the "IANA" database, since it's now being
maintained under the auspices of IANA.
600 lines
19 KiB
Plaintext
600 lines
19 KiB
Plaintext
<!-- doc/src/sgml/datetime.sgml -->
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<appendix id="datetime-appendix">
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<title>Date/Time Support</title>
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<para>
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<productname>PostgreSQL</productname> uses an internal heuristic
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parser for all date/time input support. Dates and times are input as
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strings, and are broken up into distinct fields with a preliminary
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determination of what kind of information can be in the
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field. Each field is interpreted and either assigned a numeric
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value, ignored, or rejected.
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The parser contains internal lookup tables for all textual fields,
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including months, days of the week, and time zones.
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</para>
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<para>
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This appendix includes information on the content of these
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lookup tables and describes the steps used by the parser to decode
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dates and times.
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</para>
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<sect1 id="datetime-input-rules">
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<title>Date/Time Input Interpretation</title>
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<para>
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The date/time type inputs are all decoded using the following procedure.
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</para>
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<procedure>
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<step>
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<para>
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Break the input string into tokens and categorize each token as
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a string, time, time zone, or number.
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</para>
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<substeps>
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<step>
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<para>
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If the numeric token contains a colon (<literal>:</>), this is
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a time string. Include all subsequent digits and colons.
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</para>
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</step>
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<step>
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<para>
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If the numeric token contains a dash (<literal>-</>), slash
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(<literal>/</>), or two or more dots (<literal>.</>), this is
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a date string which might have a text month. If a date token has
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already been seen, it is instead interpreted as a time zone
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name (e.g., <literal>America/New_York</>).
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</para>
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</step>
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<step>
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<para>
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If the token is numeric only, then it is either a single field
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or an ISO 8601 concatenated date (e.g.,
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<literal>19990113</literal> for January 13, 1999) or time
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(e.g., <literal>141516</literal> for 14:15:16).
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</para>
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</step>
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<step>
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<para>
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If the token starts with a plus (<literal>+</>) or minus
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(<literal>-</>), then it is either a numeric time zone or a special
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field.
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</para>
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</step>
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</substeps>
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</step>
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<step>
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<para>
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If the token is a text string, match up with possible strings:
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</para>
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<substeps>
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<step>
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<para>
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Do a binary-search table lookup for the token as a time zone
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abbreviation.
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</para>
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</step>
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<step>
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<para>
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If not found, do a similar binary-search table lookup to match
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the token as either a special string (e.g., <literal>today</literal>),
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day (e.g., <literal>Thursday</literal>),
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month (e.g., <literal>January</literal>),
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or noise word (e.g., <literal>at</literal>, <literal>on</literal>).
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</para>
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</step>
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<step>
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<para>
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If still not found, throw an error.
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</para>
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</step>
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</substeps>
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</step>
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<step>
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<para>
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When the token is a number or number field:
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</para>
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<substeps>
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<step>
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<para>
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If there are eight or six digits,
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and if no other date fields have been previously read, then interpret
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as a <quote>concatenated date</quote> (e.g.,
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<literal>19990118</literal> or <literal>990118</literal>).
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The interpretation is <literal>YYYYMMDD</> or <literal>YYMMDD</>.
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</para>
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</step>
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<step>
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<para>
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If the token is three digits
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and a year has already been read, then interpret as day of year.
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</para>
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</step>
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<step>
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<para>
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If four or six digits and a year has already been read, then
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interpret as a time (<literal>HHMM</> or <literal>HHMMSS</>).
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</para>
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</step>
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<step>
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<para>
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If three or more digits and no date fields have yet been found,
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interpret as a year (this forces yy-mm-dd ordering of the remaining
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date fields).
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</para>
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</step>
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<step>
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<para>
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Otherwise the date field ordering is assumed to follow the
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<varname>DateStyle</> setting: mm-dd-yy, dd-mm-yy, or yy-mm-dd.
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Throw an error if a month or day field is found to be out of range.
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</para>
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</step>
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</substeps>
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</step>
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<step>
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<para>
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If BC has been specified, negate the year and add one for
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internal storage. (There is no year zero in the Gregorian
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calendar, so numerically 1 BC becomes year zero.)
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</para>
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</step>
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<step>
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<para>
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If BC was not specified, and if the year field was two digits in length,
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then adjust the year to four digits. If the field is less than 70, then
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add 2000, otherwise add 1900.
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<tip>
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<para>
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Gregorian years AD 1-99 can be entered by using 4 digits with leading
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zeros (e.g., <literal>0099</> is AD 99).
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</para>
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</tip>
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</para>
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</step>
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</procedure>
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</sect1>
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<sect1 id="datetime-keywords">
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<title>Date/Time Key Words</title>
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<para>
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<xref linkend="datetime-month-table"> shows the tokens that are
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recognized as names of months.
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</para>
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<table id="datetime-month-table">
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<title>Month Names</title>
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<tgroup cols="2">
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<thead>
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<row>
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<entry>Month</entry>
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<entry>Abbreviations</entry>
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</row>
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</thead>
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<tbody>
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<row>
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<entry>January</entry>
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<entry>Jan</entry>
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</row>
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<row>
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<entry>February</entry>
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<entry>Feb</entry>
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</row>
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<row>
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<entry>March</entry>
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<entry>Mar</entry>
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</row>
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<row>
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<entry>April</entry>
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<entry>Apr</entry>
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</row>
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<row>
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<entry>May</entry>
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<entry></entry>
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</row>
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<row>
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<entry>June</entry>
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<entry>Jun</entry>
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</row>
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<row>
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<entry>July</entry>
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<entry>Jul</entry>
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</row>
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<row>
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<entry>August</entry>
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<entry>Aug</entry>
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</row>
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<row>
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<entry>September</entry>
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<entry>Sep, Sept</entry>
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</row>
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<row>
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<entry>October</entry>
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<entry>Oct</entry>
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</row>
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<row>
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<entry>November</entry>
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<entry>Nov</entry>
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</row>
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<row>
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<entry>December</entry>
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<entry>Dec</entry>
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</row>
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</tbody>
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</tgroup>
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</table>
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<para>
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<xref linkend="datetime-dow-table"> shows the tokens that are
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recognized as names of days of the week.
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</para>
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<table id="datetime-dow-table">
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<title>Day of the Week Names</title>
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<tgroup cols="2">
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<thead>
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<row>
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<entry>Day</entry>
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<entry>Abbreviations</entry>
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</row>
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</thead>
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<tbody>
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<row>
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<entry>Sunday</entry>
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<entry>Sun</entry>
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</row>
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<row>
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<entry>Monday</entry>
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<entry>Mon</entry>
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</row>
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<row>
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<entry>Tuesday</entry>
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<entry>Tue, Tues</entry>
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</row>
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<row>
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<entry>Wednesday</entry>
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<entry>Wed, Weds</entry>
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</row>
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<row>
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<entry>Thursday</entry>
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<entry>Thu, Thur, Thurs</entry>
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</row>
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<row>
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<entry>Friday</entry>
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<entry>Fri</entry>
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</row>
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<row>
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<entry>Saturday</entry>
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<entry>Sat</entry>
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</row>
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</tbody>
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</tgroup>
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</table>
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<para>
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<xref linkend="datetime-mod-table"> shows the tokens that serve
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various modifier purposes.
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</para>
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<table id="datetime-mod-table">
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<title>Date/Time Field Modifiers</title>
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<tgroup cols="2">
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<thead>
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<row>
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<entry>Identifier</entry>
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<entry>Description</entry>
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</row>
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</thead>
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<tbody>
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<row>
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<entry><literal>AM</literal></entry>
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<entry>Time is before 12:00</entry>
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</row>
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<row>
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<entry><literal>AT</literal></entry>
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<entry>Ignored</entry>
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</row>
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<row>
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<entry><literal>JULIAN</>, <literal>JD</>, <literal>J</></entry>
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<entry>Next field is Julian Date</entry>
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</row>
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<row>
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<entry><literal>ON</literal></entry>
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<entry>Ignored</entry>
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</row>
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<row>
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<entry><literal>PM</literal></entry>
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<entry>Time is on or after 12:00</entry>
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</row>
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<row>
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<entry><literal>T</literal></entry>
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<entry>Next field is time</entry>
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</row>
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</tbody>
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</tgroup>
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</table>
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</sect1>
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<sect1 id="datetime-config-files">
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<title>Date/Time Configuration Files</title>
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<indexterm>
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<primary>time zone</primary>
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<secondary>input abbreviations</secondary>
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</indexterm>
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<para>
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Since timezone abbreviations are not well standardized,
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<productname>PostgreSQL</productname> provides a means to customize
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the set of abbreviations accepted by the server. The
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<xref linkend="guc-timezone-abbreviations"> run-time parameter
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determines the active set of abbreviations. While this parameter
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can be altered by any database user, the possible values for it
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are under the control of the database administrator — they
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are in fact names of configuration files stored in
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<filename>.../share/timezonesets/</> of the installation directory.
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By adding or altering files in that directory, the administrator
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can set local policy for timezone abbreviations.
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</para>
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<para>
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<varname>timezone_abbreviations</> can be set to any file name
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found in <filename>.../share/timezonesets/</>, if the file's name
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is entirely alphabetic. (The prohibition against non-alphabetic
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characters in <varname>timezone_abbreviations</> prevents reading
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files outside the intended directory, as well as reading editor
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backup files and other extraneous files.)
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</para>
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<para>
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A timezone abbreviation file can contain blank lines and comments
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beginning with <literal>#</>. Non-comment lines must have one of
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these formats:
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<synopsis>
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<replaceable>zone_abbreviation</replaceable> <replaceable>offset</replaceable>
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<replaceable>zone_abbreviation</replaceable> <replaceable>offset</replaceable> D
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<replaceable>zone_abbreviation</replaceable> <replaceable>time_zone_name</replaceable>
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@INCLUDE <replaceable>file_name</replaceable>
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@OVERRIDE
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</synopsis>
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</para>
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<para>
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A <replaceable>zone_abbreviation</replaceable> is just the abbreviation
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being defined. The <replaceable>offset</replaceable> is the equivalent
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offset in seconds from UTC, positive being east from Greenwich and
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negative being west. For example, -18000 would be five hours west
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of Greenwich, or North American east coast standard time. <literal>D</>
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indicates that the zone name represents local daylight-savings time rather
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than standard time. Alternatively, a <replaceable>time_zone_name</> can
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be given, in which case that time zone definition is consulted, and the
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abbreviation's meaning in that zone is used. This alternative is
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recommended only for abbreviations whose meaning has historically varied,
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as looking up the meaning is noticeably more expensive than just using
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a fixed integer value.
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</para>
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<para>
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The <literal>@INCLUDE</> syntax allows inclusion of another file in the
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<filename>.../share/timezonesets/</> directory. Inclusion can be nested,
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to a limited depth.
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</para>
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<para>
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The <literal>@OVERRIDE</> syntax indicates that subsequent entries in the
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file can override previous entries (typically, entries obtained from
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included files). Without this, conflicting definitions of the same
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timezone abbreviation are considered an error.
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</para>
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<para>
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In an unmodified installation, the file <filename>Default</> contains
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all the non-conflicting time zone abbreviations for most of the world.
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Additional files <filename>Australia</> and <filename>India</> are
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provided for those regions: these files first include the
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<literal>Default</> file and then add or modify abbreviations as needed.
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</para>
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<para>
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For reference purposes, a standard installation also contains files
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<filename>Africa.txt</>, <filename>America.txt</>, etc, containing
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information about every time zone abbreviation known to be in use
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according to the IANA timezone database. The zone name
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definitions found in these files can be copied and pasted into a custom
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configuration file as needed. Note that these files cannot be directly
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referenced as <varname>timezone_abbreviations</> settings, because of
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the dot embedded in their names.
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</para>
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<note>
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<para>
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If an error occurs while reading the time zone abbreviation set, no new
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value is applied and the old set is kept. If the error occurs while
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starting the database, startup fails.
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</para>
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</note>
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<caution>
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<para>
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Time zone abbreviations defined in the configuration file override
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non-timezone meanings built into <productname>PostgreSQL</productname>.
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For example, the <filename>Australia</> configuration file defines
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<literal>SAT</> (for South Australian Standard Time). When this
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file is active, <literal>SAT</> will not be recognized as an abbreviation
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for Saturday.
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</para>
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</caution>
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<caution>
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<para>
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If you modify files in <filename>.../share/timezonesets/</>,
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it is up to you to make backups — a normal database dump
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will not include this directory.
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</para>
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</caution>
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</sect1>
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<sect1 id="datetime-units-history">
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<title>History of Units</title>
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<indexterm zone="datetime-units-history">
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<primary>Gregorian calendar</primary>
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</indexterm>
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<indexterm zone="datetime-units-history">
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<primary>Julian date</primary>
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</indexterm>
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<para>
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The SQL standard states that <quote>Within the definition of a
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<quote>datetime literal</quote>, the <quote>datetime
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values</quote> are constrained by the natural rules for dates and
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times according to the Gregorian calendar</quote>.
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<productname>PostgreSQL</> follows the SQL
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standard's lead by counting dates exclusively in the Gregorian
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calendar, even for years before that calendar was in use.
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This rule is known as the <firstterm>proleptic Gregorian calendar</>.
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</para>
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<para>
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The Julian calendar was introduced by Julius Caesar in 45 BC.
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It was in common use in the Western world
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until the year 1582, when countries started changing to the Gregorian
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calendar. In the Julian calendar, the tropical year is
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approximated as 365 1/4 days = 365.25 days. This gives an error of
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about 1 day in 128 years.
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</para>
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<para>
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The accumulating calendar error prompted
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Pope Gregory XIII to reform the calendar in accordance with
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instructions from the Council of Trent.
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In the Gregorian calendar, the tropical year is approximated as
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365 + 97 / 400 days = 365.2425 days. Thus it takes approximately 3300
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years for the tropical year to shift one day with respect to the
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Gregorian calendar.
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</para>
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<para>
|
|
The approximation 365+97/400 is achieved by having 97 leap years
|
|
every 400 years, using the following rules:
|
|
|
|
<simplelist>
|
|
<member>
|
|
Every year divisible by 4 is a leap year.
|
|
</member>
|
|
<member>
|
|
However, every year divisible by 100 is not a leap year.
|
|
</member>
|
|
<member>
|
|
However, every year divisible by 400 is a leap year after all.
|
|
</member>
|
|
</simplelist>
|
|
|
|
So, 1700, 1800, 1900, 2100, and 2200 are not leap years. But 1600,
|
|
2000, and 2400 are leap years.
|
|
|
|
By contrast, in the older Julian calendar all years divisible by 4 are leap
|
|
years.
|
|
</para>
|
|
|
|
<para>
|
|
The papal bull of February 1582 decreed that 10 days should be dropped
|
|
from October 1582 so that 15 October should follow immediately after
|
|
4 October.
|
|
This was observed in Italy, Poland, Portugal, and Spain. Other Catholic
|
|
countries followed shortly after, but Protestant countries were
|
|
reluctant to change, and the Greek Orthodox countries didn't change
|
|
until the start of the 20th century.
|
|
|
|
The reform was observed by Great Britain and its dominions (including what
|
|
is now the USA) in 1752.
|
|
Thus 2 September 1752 was followed by 14 September 1752.
|
|
|
|
This is why Unix systems have the <command>cal</command> program
|
|
produce the following:
|
|
|
|
<screen>
|
|
$ <userinput>cal 9 1752</userinput>
|
|
September 1752
|
|
S M Tu W Th F S
|
|
1 2 14 15 16
|
|
17 18 19 20 21 22 23
|
|
24 25 26 27 28 29 30
|
|
</screen>
|
|
|
|
But, of course, this calendar is only valid for Great Britain and
|
|
dominions, not other places.
|
|
Since it would be difficult and confusing to try to track the actual
|
|
calendars that were in use in various places at various times,
|
|
<productname>PostgreSQL</> does not try, but rather follows the Gregorian
|
|
calendar rules for all dates, even though this method is not historically
|
|
accurate.
|
|
</para>
|
|
|
|
<para>
|
|
Different calendars have been developed in various parts of the
|
|
world, many predating the Gregorian system.
|
|
|
|
For example,
|
|
the beginnings of the Chinese calendar can be traced back to the 14th
|
|
century BC. Legend has it that the Emperor Huangdi invented that
|
|
calendar in 2637 BC.
|
|
|
|
The People's Republic of China uses the Gregorian calendar
|
|
for civil purposes. The Chinese calendar is used for determining
|
|
festivals.
|
|
</para>
|
|
|
|
<para>
|
|
The <firstterm>Julian Date</firstterm> system is another type of
|
|
calendar, unrelated to the Julian calendar though it is confusingly
|
|
named similarly to that calendar.
|
|
The Julian Date system was invented by the French scholar
|
|
Joseph Justus Scaliger (1540-1609)
|
|
and probably takes its name from Scaliger's father,
|
|
the Italian scholar Julius Caesar Scaliger (1484-1558).
|
|
In the Julian Date system, each day has a sequential number, starting
|
|
from JD 0 (which is sometimes called <emphasis>the</> Julian Date).
|
|
JD 0 corresponds to 1 January 4713 BC in the Julian calendar, or
|
|
24 November 4714 BC in the Gregorian calendar. Julian Date counting
|
|
is most often used by astronomers for labeling their nightly observations,
|
|
and therefore a date runs from noon UTC to the next noon UTC, rather than
|
|
from midnight to midnight: JD 0 designates the 24 hours from noon UTC on
|
|
24 November 4714 BC to noon UTC on 25 November 4714 BC.
|
|
</para>
|
|
|
|
<para>
|
|
Although <productname>PostgreSQL</> supports Julian Date notation for
|
|
input and output of dates (and also uses Julian dates for some internal
|
|
datetime calculations), it does not observe the nicety of having dates
|
|
run from noon to noon. <productname>PostgreSQL</> treats a Julian Date
|
|
as running from midnight to midnight.
|
|
</para>
|
|
|
|
</sect1>
|
|
</appendix>
|