619 lines
20 KiB
Plaintext
619 lines
20 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. An <replaceable>offset</replaceable> is an integer giving
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the equivalent offset in seconds from UTC, positive being east from
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Greenwich and negative being west. For example, -18000 would be five
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hours west of Greenwich, or North American east coast standard time.
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<literal>D</> indicates that the zone name represents local
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daylight-savings time rather than standard time.
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</para>
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<para>
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Alternatively, a <replaceable>time_zone_name</> can be given, referencing
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a zone name defined in the IANA timezone database. The zone's definition
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is consulted to see whether the abbreviation is or has been in use in
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that zone, and if so, the appropriate meaning is used — that is,
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the meaning that was currently in use at the timestamp whose value is
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being determined, or the meaning in use immediately before that if it
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wasn't current at that time, or the oldest meaning if it was used only
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after that time. This behavior is essential for dealing with
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abbreviations whose meaning has historically varied. It is also allowed
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to define an abbreviation in terms of a zone name in which that
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abbreviation does not appear; then using the abbreviation is just
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equivalent to writing out the zone name.
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</para>
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<tip>
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<para>
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Using a simple integer <replaceable>offset</replaceable> is preferred
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when defining an abbreviation whose offset from UTC has never changed,
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as such abbreviations are much cheaper to process than those that
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require consulting a time zone definition.
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</para>
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</tip>
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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>
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The approximation 365+97/400 is achieved by having 97 leap years
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every 400 years, using the following rules:
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<simplelist>
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<member>
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Every year divisible by 4 is a leap year.
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</member>
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<member>
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However, every year divisible by 100 is not a leap year.
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</member>
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<member>
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However, every year divisible by 400 is a leap year after all.
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</member>
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</simplelist>
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So, 1700, 1800, 1900, 2100, and 2200 are not leap years. But 1600,
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2000, and 2400 are leap years.
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By contrast, in the older Julian calendar all years divisible by 4 are leap
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years.
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</para>
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<para>
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The papal bull of February 1582 decreed that 10 days should be dropped
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from October 1582 so that 15 October should follow immediately after
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4 October.
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This was observed in Italy, Poland, Portugal, and Spain. Other Catholic
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countries followed shortly after, but Protestant countries were
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reluctant to change, and the Greek Orthodox countries didn't change
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until the start of the 20th century.
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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>
|