-
Tom Lane authored
input routines. Remove the former "DecodePosixTimezone" function in favor of letting the zic code handle POSIX-style zone specs (see tzparse()). In particular this means that "PST+3" now means the same as "-03", whereas it used to mean "-11" --- the zone abbreviation is effectively just a noise word in this syntax. Make sure that all named and POSIX-style zone names will be parsed as a single token. Fix long-standing bogosities in printing and input of fractional-hour timezone offsets (since the tzparse() code will accept these, we'd better make 'em work). Also correct an error in the original coding of the zic-zone-name patch: in "timestamp without time zone" input, zone names are supposed to be allowed but ignored, but the coding was such that the zone changed the interpretation anyway.
Tom Lane authoredinput routines. Remove the former "DecodePosixTimezone" function in favor of letting the zic code handle POSIX-style zone specs (see tzparse()). In particular this means that "PST+3" now means the same as "-03", whereas it used to mean "-11" --- the zone abbreviation is effectively just a noise word in this syntax. Make sure that all named and POSIX-style zone names will be parsed as a single token. Fix long-standing bogosities in printing and input of fractional-hour timezone offsets (since the tzparse() code will accept these, we'd better make 'em work). Also correct an error in the original coding of the zic-zone-name patch: in "timestamp without time zone" input, zone names are supposed to be allowed but ignored, but the coding was such that the zone changed the interpretation anyway.
datetime.sgml 17.44 KiB
<!-- $PostgreSQL: pgsql/doc/src/sgml/datetime.sgml,v 2.55 2006/10/17 21:03:20 tgl Exp $ -->
<appendix id="datetime-appendix">
<title>Date/Time Support</title>
<para>
<productname>PostgreSQL</productname> uses an internal heuristic
parser for all date/time input support. Dates and times are input as
strings, and are broken up into distinct fields with a preliminary
determination of what kind of information may be in the
field. Each field is interpreted and either assigned a numeric
value, ignored, or rejected.
The parser contains internal lookup tables for all textual fields,
including months, days of the week, and time zones.
</para>
<para>
This appendix includes information on the content of these
lookup tables and describes the steps used by the parser to decode
dates and times.
</para>
<sect1>
<title>Date/Time Input Interpretation</title>
<para>
The date/time type inputs are all decoded using the following procedure.
</para>
<procedure>
<step>
<para>
Break the input string into tokens and categorize each token as
a string, time, time zone, or number.
</para>
<substeps>
<step>
<para>
If the numeric token contains a colon (<literal>:</>), this is
a time string. Include all subsequent digits and colons.
</para>
</step>
<step>
<para>
If the numeric token contains a dash (<literal>-</>), slash
(<literal>/</>), or two or more dots (<literal>.</>), this is
a date string which may have a text month. If a date token has
already been seen, it is instead interpreted as a time zone
name (e.g., <literal>America/New_York</>).
</para>
</step>
<step>
<para>
If the token is numeric only, then it is either a single field
or an ISO 8601 concatenated date (e.g.,
<literal>19990113</literal> for January 13, 1999) or time
(e.g., <literal>141516</literal> for 14:15:16).
</para>
</step>
<step>
<para>
If the token starts with a plus (<literal>+</>) or minus
(<literal>-</>), then it is either a numeric time zone or a special
field.
</para>
</step> </substeps>
</step>
<step>
<para>
If the token is a text string, match up with possible strings:
</para>
<substeps>
<step>
<para>
Do a binary-search table lookup for the token as a time zone
abbreviation.
</para>
</step>
<step>
<para>
If not found, do a similar binary-search table lookup to match
the token as either a special string (e.g., <literal>today</literal>),
day (e.g., <literal>Thursday</literal>),
month (e.g., <literal>January</literal>),
or noise word (e.g., <literal>at</literal>, <literal>on</literal>).
</para>
</step>
<step>
<para>
If still not found, throw an error.
</para>
</step>
</substeps>
</step>
<step>
<para>
When the token is a number or number field:
</para>
<substeps>
<step>
<para>
If there are eight or six digits,
and if no other date fields have been previously read, then interpret
as a <quote>concatenated date</quote> (e.g.,
<literal>19990118</literal> or <literal>990118</literal>).
The interpretation is <literal>YYYYMMDD</> or <literal>YYMMDD</>.
</para>
</step>
<step>
<para>
If the token is three digits
and a year has already been read, then interpret as day of year.
</para>
</step>
<step>
<para>
If four or six digits and a year has already been read, then
interpret as a time (<literal>HHMM</> or <literal>HHMMSS</>).
</para>
</step>
<step>
<para>
If three or more digits and no date fields have yet been found,
interpret as a year (this forces yy-mm-dd ordering of the remaining
date fields).
</para> </step>
<step>
<para>
Otherwise the date field ordering is assumed to follow the
<varname>DateStyle</> setting: mm-dd-yy, dd-mm-yy, or yy-mm-dd.
Throw an error if a month or day field is found to be out of range.
</para>
</step>
</substeps>
</step>
<step>
<para>
If BC has been specified, negate the year and add one for
internal storage. (There is no year zero in the Gregorian
calendar, so numerically 1 BC becomes year zero.)
</para>
</step>
<step>
<para>
If BC was not specified, and if the year field was two digits in length,
then adjust the year to four digits. If the field is less than 70, then
add 2000, otherwise add 1900.
<tip>
<para>
Gregorian years AD 1-99 may be entered by using 4 digits with leading
zeros (e.g., <literal>0099</> is AD 99).
</para>
</tip>
</para>
</step>
</procedure>
</sect1>
<sect1 id="datetime-keywords">
<title>Date/Time Key Words</title>
<para>
<xref linkend="datetime-month-table"> shows the tokens that are
recognized as names of months.
</para>
<table id="datetime-month-table">
<title>Month Names</title>
<tgroup cols="2">
<thead>
<row>
<entry>Month</entry>
<entry>Abbreviations</entry>
</row>
</thead>
<tbody>
<row>
<entry>January</entry>
<entry>Jan</entry>
</row>
<row>
<entry>February</entry>
<entry>Feb</entry>
</row>
<row>
<entry>March</entry>
<entry>Mar</entry>
</row>
<row>
<entry>April</entry> <entry>Apr</entry>
</row>
<row>
<entry>May</entry>
<entry></entry>
</row>
<row>
<entry>June</entry>
<entry>Jun</entry>
</row>
<row>
<entry>July</entry>
<entry>Jul</entry>
</row>
<row>
<entry>August</entry>
<entry>Aug</entry>
</row>
<row>
<entry>September</entry>
<entry>Sep, Sept</entry>
</row>
<row>
<entry>October</entry>
<entry>Oct</entry>
</row>
<row>
<entry>November</entry>
<entry>Nov</entry>
</row>
<row>
<entry>December</entry>
<entry>Dec</entry>
</row>
</tbody>
</tgroup>
</table>
<para>
<xref linkend="datetime-dow-table"> shows the tokens that are
recognized as names of days of the week.
</para>
<table id="datetime-dow-table">
<title>Day of the Week Names</title>
<tgroup cols="2">
<thead>
<row>
<entry>Day</entry>
<entry>Abbreviations</entry>
</row>
</thead>
<tbody>
<row>
<entry>Sunday</entry>
<entry>Sun</entry>
</row>
<row>
<entry>Monday</entry>
<entry>Mon</entry>
</row>
<row>
<entry>Tuesday</entry>
<entry>Tue, Tues</entry>
</row>
<row>
<entry>Wednesday</entry>
<entry>Wed, Weds</entry>
</row>
<row> <entry>Thursday</entry>
<entry>Thu, Thur, Thurs</entry>
</row>
<row>
<entry>Friday</entry>
<entry>Fri</entry>
</row>
<row>
<entry>Saturday</entry>
<entry>Sat</entry>
</row>
</tbody>
</tgroup>
</table>
<para>
<xref linkend="datetime-mod-table"> shows the tokens that serve
various modifier purposes.
</para>
<table id="datetime-mod-table">
<title>Date/Time Field Modifiers</title>
<tgroup cols="2">
<thead>
<row>
<entry>Identifier</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry><literal>ABSTIME</literal></entry>
<entry>Ignored</entry>
</row>
<row>
<entry><literal>AM</literal></entry>
<entry>Time is before 12:00</entry>
</row>
<row>
<entry><literal>AT</literal></entry>
<entry>Ignored</entry>
</row>
<row>
<entry><literal>JULIAN</>, <literal>JD</>, <literal>J</></entry>
<entry>Next field is Julian Day</entry>
</row>
<row>
<entry><literal>ON</literal></entry>
<entry>Ignored</entry>
</row>
<row>
<entry><literal>PM</literal></entry>
<entry>Time is on or after 12:00</entry>
</row>
<row>
<entry><literal>T</literal></entry>
<entry>Next field is time</entry>
</row>
</tbody>
</tgroup>
</table>
<para>
The key word <literal>ABSTIME</literal> is ignored for historical
reasons: In very old releases of
<productname>PostgreSQL</productname>, invalid values of type <type>abstime</type>
were emitted as <literal>Invalid Abstime</literal>. This is no
longer the case however and this key word will likely be dropped in
a future release.
</para> </sect1>
<sect1 id="datetime-config-files">
<title>Date/Time Configuration Files</title>
<indexterm>
<primary>time zone</primary>
<secondary>input abbreviations</secondary>
</indexterm>
<para>
Since timezone abbreviations are not well standardized,
<productname>PostgreSQL</productname> provides a means to customize
the set of abbreviations accepted by the server. The
<xref linkend="guc-timezone-abbreviations"> run-time parameter
determines the active set of abbreviations. While this parameter
can be altered by any database user, the possible values for it
are under the control of the database administrator — they
are in fact names of configuration files stored in
<filename>.../share/timezonesets/</> of the installation directory.
By adding or altering files in that directory, the administrator
can set local policy for timezone abbreviations.
</para>
<para>
<literal>timezone_abbreviations</> can be set to any file name
found in <filename>.../share/timezonesets/</>, if the file's name
is entirely alphabetic. (The prohibition against non-alphabetic
characters in <literal>timezone_abbreviations</> prevents reading
files outside the intended directory, as well as reading editor
backup files and other extraneous files.)
</para>
<para>
A timezone abbreviation file may contain blank lines and comments
beginning with <literal>#</>. Non-comment lines must have one of
these formats:
<synopsis>
<replaceable>time_zone_name</replaceable> <replaceable>offset</replaceable>
<replaceable>time_zone_name</replaceable> <replaceable>offset</replaceable> D
@INCLUDE <replaceable>file_name</replaceable>
@OVERRIDE
</synopsis>
</para>
<para>
A <replaceable>time_zone_name</replaceable> is just the abbreviation
being defined. The <replaceable>offset</replaceable> is the zone's
offset in seconds from UTC, positive being east from Greenwich and
negative being west. For example, -18000 would be five hours west
of Greenwich, or North American east coast standard time. <literal>D</>
indicates that the zone name represents local daylight-savings time
rather than standard time. Since all known time zone offsets are on
15 minute boundaries, the number of seconds has to be a multiple of 900.
</para>
<para>
The <literal>@INCLUDE</> syntax allows inclusion of another file in the
<filename>.../share/timezonesets/</> directory. Inclusion can be nested,
to a limited depth.
</para>
<para>
The <literal>@OVERRIDE</> syntax indicates that subsequent entries in the
file may override previous entries (i.e., entries obtained from included
files). Without this, conflicting definitions of the same timezone
abbreviation are considered an error.
</para>
<para>
In an unmodified installation, the file <filename>Default</> contains
all the non-conflicting time zone abbreviations for most of the world.
Additional files <filename>Australia</> and <filename>India</> are
provided for those regions: these files first include the
<literal>Default</> file and then add or modify timezones as needed.
</para>
<para>
For reference purposes, a standard installation also contains files
<filename>Africa.txt</>, <filename>America.txt</>, etc, containing
information about every time zone abbreviation known to be in use
according to the <literal>zic</> timezone database. The zone name
definitions found in these files can be copied and pasted into a custom
configuration file as needed. Note that these files cannot be directly
referenced as <literal>timezone_abbreviations</> settings, because of
the dot embedded in their names.
</para>
<note>
<para>
If an error occurs while reading the time zone data sets, no new value is
applied but the old set is kept. If the error occurs while starting the
database, startup fails.
</para>
</note>
<caution>
<para>
Time zone abbreviations defined in the configuration file override
non-timezone meanings built into <productname>PostgreSQL</productname>.
For example, the <filename>Australia</> configuration file defines
<literal>SAT</> (for South Australian Standard Time). When this
file is active, <literal>SAT</> will not be recognized as an abbreviation
for Saturday.
</para>
</caution>
<caution>
<para>
If you modify files in <filename>.../share/timezonesets/</>,
it is up to you to make backups — a normal database dump
will not include this directory.
</para>
</caution>
</sect1>
<sect1 id="datetime-units-history">
<title>History of Units</title>
<para>
The Julian Date 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).
Astronomers have used the Julian period to assign a unique number to
every day since 1 January 4713 BC. This is the so-called Julian Date
(JD). JD 0 designates the 24 hours from noon UTC on 1 January 4713 BC
to noon UTC on 2 January 4713 BC.
</para>
<para>
The <quote>Julian Date</quote> is different from the <quote>Julian
Calendar</quote>. The Julian calendar
was introduced by Julius Caesar in 45 BC. It was in common use
until the year 1582, when countries started changing to the Gregorian
calendar. In the Julian calendar, the tropical year is
approximated as 365 1/4 days = 365.25 days. This gives an error of
about 1 day in 128 years. </para>
<para>
The accumulating calendar error prompted
Pope Gregory XIII to reform the calendar in accordance with
instructions from the Council of Trent.
In the Gregorian calendar, the tropical year is approximated as
365 + 97 / 400 days = 365.2425 days. Thus it takes approximately 3300
years for the tropical year to shift one day with respect to the
Gregorian calendar.
</para>
<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 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>
</para>
<note>
<para>
The SQL standard states that <quote>Within the definition of a
<quote>datetime literal</quote>, the <quote>datetime
value</quote>s are constrained by the natural rules for dates and
times according to the Gregorian calendar</quote>. Dates between
1752-09-03 and 1752-09-13, although eliminated in some countries
by Papal fiat, conform to <quote>natural rules</quote> and are
hence valid dates. </para>
</note>
<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 the
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>
</sect1>
</appendix>