diff options
Diffstat (limited to 'doc/src/sgml/datetime.sgml')
| -rw-r--r-- | doc/src/sgml/datetime.sgml | 930 |
1 files changed, 930 insertions, 0 deletions
diff --git a/doc/src/sgml/datetime.sgml b/doc/src/sgml/datetime.sgml new file mode 100644 index 0000000..ecc3245 --- /dev/null +++ b/doc/src/sgml/datetime.sgml @@ -0,0 +1,930 @@ +<!-- doc/src/sgml/datetime.sgml --> + + <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 can 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 id="datetime-input-rules"> + <title>Date/Time Input Interpretation</title> + + <para> + Date/time input strings are 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>:</literal>), this is + a time string. Include all subsequent digits and colons. + </para> + </step> + + <step> + <para> + If the numeric token contains a dash (<literal>-</literal>), slash + (<literal>/</literal>), or two or more dots (<literal>.</literal>), this is + a date string which might 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</literal>). + </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>+</literal>) or minus + (<literal>-</literal>), then it is either a numeric time zone or a special + field. + </para> + </step> + </substeps> + </step> + + <step> + <para> + If the token is an alphabetic string, match up with possible strings: + </para> + + <substeps> + <step> + <para> + See if the token matches any known time zone abbreviation. + These abbreviations are supplied by the configuration file + described in <xref linkend="datetime-config-files"/>. + </para> + </step> + + <step> + <para> + If not found, search an internal table 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</literal> or <literal>YYMMDD</literal>. + </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</literal> or <literal>HHMMSS</literal>). + </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</varname> 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 can be entered by using 4 digits with leading + zeros (e.g., <literal>0099</literal> is AD 99). + </para> + </tip> + </para> + </step> + </procedure> + </sect1> + + + <sect1 id="datetime-invalid-input"> + <title>Handling of Invalid or Ambiguous Timestamps</title> + + <para> + 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. + </para> + + <para> + 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 + <xref linkend="guc-timezone"/> parameter is set + to <literal>America/New_York</literal>, consider +<programlisting> +=> SELECT '2018-03-11 02:30'::timestamptz; + timestamptz +------------------------ + 2018-03-11 03:30:00-04 +(1 row) +</programlisting> + 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. <productname>PostgreSQL</productname> interprets the + given time as if it were standard time (UTC-5), which then renders as + 3:30AM EDT (UTC-4). + </para> + + <para> + Conversely, consider the behavior during a fall-back transition: +<programlisting> +=> SELECT '2018-11-04 01:30'::timestamptz; + timestamptz +------------------------ + 2018-11-04 01:30:00-05 +(1 row) +</programlisting> + On that date, there were two possible interpretations of 1:30AM; there + was 1:30AM EDT, and then an hour later after clocks jumped back from + 2AM EDT to 1AM EST, there was 1:30AM EST. + Again, <productname>PostgreSQL</productname> interprets the given time + as if it were standard time (UTC-5). We can force the other + interpretation by specifying daylight-savings time: +<programlisting> +=> SELECT '2018-11-04 01:30 EDT'::timestamptz; + timestamptz +------------------------ + 2018-11-04 01:30:00-04 +(1 row) +</programlisting> + </para> + + <para> + 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 <quote>the standard-time interpretation is + preferred when in doubt</quote>. + </para> + + <para> + 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. + </para> + </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>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>, <literal>JD</literal>, <literal>J</literal></entry> + <entry>Next field is Julian Date</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> + </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/</filename> of the installation directory. + By adding or altering files in that directory, the administrator + can set local policy for timezone abbreviations. + </para> + + <para> + <varname>timezone_abbreviations</varname> can be set to any file name + found in <filename>.../share/timezonesets/</filename>, if the file's name + is entirely alphabetic. (The prohibition against non-alphabetic + characters in <varname>timezone_abbreviations</varname> prevents reading + files outside the intended directory, as well as reading editor + backup files and other extraneous files.) + </para> + + <para> + A timezone abbreviation file can contain blank lines and comments + beginning with <literal>#</literal>. Non-comment lines must have one of + these formats: + +<synopsis> +<replaceable>zone_abbreviation</replaceable> <replaceable>offset</replaceable> +<replaceable>zone_abbreviation</replaceable> <replaceable>offset</replaceable> D +<replaceable>zone_abbreviation</replaceable> <replaceable>time_zone_name</replaceable> +@INCLUDE <replaceable>file_name</replaceable> +@OVERRIDE +</synopsis> + </para> + + <para> + A <replaceable>zone_abbreviation</replaceable> is just the abbreviation + being defined. An <replaceable>offset</replaceable> is an integer giving + the equivalent 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</literal> indicates that the zone name represents local + daylight-savings time rather than standard time. + </para> + + <para> + Alternatively, a <replaceable>time_zone_name</replaceable> can be given, referencing + a zone name defined in the IANA timezone database. The zone's definition + is consulted to see whether the abbreviation is or has been in use in + that zone, and if so, the appropriate meaning is used — that is, + the meaning that was currently in use at the timestamp whose value is + being determined, or the meaning in use immediately before that if it + wasn't current at that time, or the oldest meaning if it was used only + after that time. This behavior is essential for dealing with + abbreviations whose meaning has historically varied. It is also allowed + to define an abbreviation in terms of a zone name in which that + abbreviation does not appear; then using the abbreviation is just + equivalent to writing out the zone name. + </para> + + <tip> + <para> + Using a simple integer <replaceable>offset</replaceable> is preferred + when defining an abbreviation whose offset from UTC has never changed, + as such abbreviations are much cheaper to process than those that + require consulting a time zone definition. + </para> + </tip> + + <para> + The <literal>@INCLUDE</literal> syntax allows inclusion of another file in the + <filename>.../share/timezonesets/</filename> directory. Inclusion can be nested, + to a limited depth. + </para> + + <para> + The <literal>@OVERRIDE</literal> syntax indicates that subsequent entries in the + file can override previous entries (typically, 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</filename> contains + all the non-conflicting time zone abbreviations for most of the world. + Additional files <filename>Australia</filename> and <filename>India</filename> are + provided for those regions: these files first include the + <literal>Default</literal> file and then add or modify abbreviations as needed. + </para> + + <para> + For reference purposes, a standard installation also contains files + <filename>Africa.txt</filename>, <filename>America.txt</filename>, etc., containing + information about every time zone abbreviation known to be in use + according to the IANA 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 <varname>timezone_abbreviations</varname> settings, because of + the dot embedded in their names. + </para> + + <note> + <para> + If an error occurs while reading the time zone abbreviation set, no new + value is applied and 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</filename> configuration file defines + <literal>SAT</literal> (for South Australian Standard Time). When this + file is active, <literal>SAT</literal> will not be recognized as an abbreviation + for Saturday. + </para> + </caution> + + <caution> + <para> + If you modify files in <filename>.../share/timezonesets/</filename>, + it is up to you to make backups — a normal database dump + will not include this directory. + </para> + </caution> + + </sect1> + + <sect1 id="datetime-posix-timezone-specs"> + <title><acronym>POSIX</acronym> Time Zone Specifications</title> + + <indexterm zone="datetime-posix-timezone-specs"> + <primary>time zone</primary> + <secondary><acronym>POSIX</acronym>-style specification</secondary> + </indexterm> + + <para> + <productname>PostgreSQL</productname> can accept time zone specifications + that are written according to the <acronym>POSIX</acronym> standard's rules + for the <varname>TZ</varname> environment + variable. <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. + </para> + + <para> + A POSIX time zone specification has the form +<synopsis> +<replaceable>STD</replaceable> <replaceable>offset</replaceable> <optional> <replaceable>DST</replaceable> <optional> <replaceable>dstoffset</replaceable> </optional> <optional> , <replaceable>rule</replaceable> </optional> </optional> +</synopsis> + (For readability, we show spaces between the fields, but spaces should + not be used in practice.) The fields are: + <itemizedlist> + <listitem> + <para> + <replaceable>STD</replaceable> is the zone abbreviation to be used + for standard time. + </para> + </listitem> + <listitem> + <para> + <replaceable>offset</replaceable> is the zone's standard-time offset + from UTC. + </para> + </listitem> + <listitem> + <para> + <replaceable>DST</replaceable> 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. + </para> + </listitem> + <listitem> + <para> + <replaceable>dstoffset</replaceable> is the daylight-savings offset + from UTC. This field is typically omitted, since it defaults to one + hour less than the standard-time <replaceable>offset</replaceable>, + which is usually the right thing. + </para> + </listitem> + <listitem> + <para> + <replaceable>rule</replaceable> defines the rule for when daylight + savings is in effect, as described below. + </para> + </listitem> + </itemizedlist> + </para> + + <para> + In this syntax, a zone abbreviation can be a string of letters, such + as <literal>EST</literal>, or an arbitrary string surrounded by angle + brackets, such as <literal><UTC-05></literal>. + 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 <xref linkend="datetime-config-files"/>. + </para> + + <para> + The offset fields specify the hours, and optionally minutes and seconds, + difference from UTC. They have the format + <replaceable>hh</replaceable><optional><literal>:</literal><replaceable>mm</replaceable><optional><literal>:</literal><replaceable>ss</replaceable></optional></optional> + optionally with a leading sign (<literal>+</literal> + or <literal>-</literal>). The positive sign is used for + zones <emphasis>west</emphasis> of Greenwich. (Note that this is the + opposite of the ISO-8601 sign convention used elsewhere in + <productname>PostgreSQL</productname>.) <replaceable>hh</replaceable> + can have one or two digits; <replaceable>mm</replaceable> + and <replaceable>ss</replaceable> (if used) must have two. + </para> + + <para> + The daylight-savings transition <replaceable>rule</replaceable> has the + format +<synopsis> +<replaceable>dstdate</replaceable> <optional> <literal>/</literal> <replaceable>dsttime</replaceable> </optional> <literal>,</literal> <replaceable>stddate</replaceable> <optional> <literal>/</literal> <replaceable>stdtime</replaceable> </optional> +</synopsis> + (As before, spaces should not be included in practice.) + The <replaceable>dstdate</replaceable> + and <replaceable>dsttime</replaceable> fields define when daylight-savings + time starts, while <replaceable>stddate</replaceable> + and <replaceable>stdtime</replaceable> 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: + <variablelist> + <varlistentry> + <term><replaceable>n</replaceable></term> + <listitem> + <para> + A plain integer denotes a day of the year, counting from zero to + 364, or to 365 in leap years. + </para> + </listitem> + </varlistentry> + <varlistentry> + <term><literal>J</literal><replaceable>n</replaceable></term> + <listitem> + <para> + In this form, <replaceable>n</replaceable> 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.) + </para> + </listitem> + </varlistentry> + <varlistentry> + <term><literal>M</literal><replaceable>m</replaceable><literal>.</literal><replaceable>n</replaceable><literal>.</literal><replaceable>d</replaceable></term> + <listitem> + <para> + This form specifies a transition that always happens during the same + month and on the same day of the week. <replaceable>m</replaceable> + identifies the month, from 1 to 12. <replaceable>n</replaceable> + specifies the <replaceable>n</replaceable>'th occurrence of the + weekday identified by <replaceable>d</replaceable>. + <replaceable>n</replaceable> 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). <replaceable>d</replaceable> is + a number between 0 and 6, with 0 indicating Sunday. + For example, <literal>M3.2.0</literal> means <quote>the second + Sunday in March</quote>. + </para> + </listitem> + </varlistentry> + </variablelist> + </para> + + <note> + <para> + The <literal>M</literal> 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. + </para> + </note> + + <para> + 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 <literal>02:00:00</literal>. + </para> + + <para> + If a daylight-savings abbreviation is given but the + transition <replaceable>rule</replaceable> field is omitted, + the fallback behavior is to use the + rule <literal>M3.2.0,M11.1.0</literal>, 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. + </para> + + <para> + As an example, <literal>CET-1CEST,M3.5.0,M10.5.0/3</literal> describes + current (as of 2020) timekeeping practice in Paris. This specification + says that standard time has the abbreviation <literal>CET</literal> and + is one hour ahead (east) of UTC; daylight savings time has the + abbreviation <literal>CEST</literal> 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. + </para> + + <para> + The four timezone names <literal>EST5EDT</literal>, + <literal>CST6CDT</literal>, <literal>MST7MDT</literal>, + and <literal>PST8PDT</literal> 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. + </para> + + <para> + 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, <literal>SET TIMEZONE TO + FOOBAR0</literal> will work, leaving the system effectively using a + rather peculiar abbreviation for UTC. + </para> + + </sect1> + + <sect1 id="datetime-units-history"> + <title>History of Units</title> + + <indexterm zone="datetime-units-history"> + <primary>Gregorian calendar</primary> + </indexterm> + + <para> + The SQL standard states that <quote>Within the definition of a + <quote>datetime literal</quote>, the <quote>datetime + values</quote> are constrained by the natural rules for dates and + times according to the Gregorian calendar</quote>. + <productname>PostgreSQL</productname> follows the SQL + standard's lead by counting dates exclusively in the Gregorian + calendar, even for years before that calendar was in use. + This rule is known as the <firstterm>proleptic Gregorian calendar</firstterm>. + </para> + + <para> + The Julian calendar was introduced by Julius Caesar in 45 BC. + It was in common use in the Western world + 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 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 that 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</productname> 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> + + </sect1> + + <sect1 id="datetime-julian-dates"> + <title>Julian Dates</title> + + <indexterm zone="datetime-julian-dates"> + <primary>Julian date</primary> + </indexterm> + + <para> + The <firstterm>Julian Date</firstterm> system is a method for + numbering days. It is + 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). + </para> + + <para> + In the Julian Date system, each day has a sequential number, starting + from JD 0 (which is sometimes called <emphasis>the</emphasis> 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</productname> 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</productname> treats a Julian Date + as running from local midnight to local midnight, the same as a normal + date. + </para> + + <para> + This definition does, however, provide a way to obtain the astronomical + definition when you need it: do the arithmetic in time + zone <literal>UTC+12</literal>. For example, +<programlisting> +=> SELECT extract(julian from '2021-06-23 7:00:00-04'::timestamptz at time zone 'UTC+12'); + extract +------------------------------ + 2459388.95833333333333333333 +(1 row) +=> SELECT extract(julian from '2021-06-23 8:00:00-04'::timestamptz at time zone 'UTC+12'); + extract +-------------------------------------- + 2459389.0000000000000000000000000000 +(1 row) +=> SELECT extract(julian from date '2021-06-23'); + extract +--------- + 2459389 +(1 row) +</programlisting> + </para> + + </sect1> +</appendix> |