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>Appendix B. Date/Time Support</H1
><DIV
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><DL
><DT
><B
>Table of Contents</B
></DT
><DT
>B.1. <A
HREF="datetime-appendix.html#AEN59479"
>Date/Time Input Interpretation</A
></DT
><DT
>B.2. <A
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>Date/Time Key Words</A
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>B.3. <A
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>History of Units</A
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><P
> <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> 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.
</P
><P
> This appendix includes information on the content of these
lookup tables and describes the steps used by the parser to decode
dates and times.
</P
><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="AEN59479"
>B.1. Date/Time Input Interpretation</A
></H1
><P
> The date/time type inputs are all decoded using the following procedure.
</P
><DIV
CLASS="PROCEDURE"
><OL
TYPE="1"
><LI
CLASS="STEP"
><P
> Break the input string into tokens and categorize each token as
a string, time, time zone, or number.
</P
><OL
CLASS="SUBSTEPS"
TYPE="a"
><LI
CLASS="STEP"
><P
> If the numeric token contains a colon (<TT
CLASS="LITERAL"
>:</TT
>), this is
a time string. Include all subsequent digits and colons.
</P
></LI
><LI
CLASS="STEP"
><P
> If the numeric token contains a dash (<TT
CLASS="LITERAL"
>-</TT
>), slash
(<TT
CLASS="LITERAL"
>/</TT
>), or two or more dots (<TT
CLASS="LITERAL"
>.</TT
>), this is
a date string which may have a text month.
</P
></LI
><LI
CLASS="STEP"
><P
> If the token is numeric only, then it is either a single field
or an ISO 8601 concatenated date (e.g.,
<TT
CLASS="LITERAL"
>19990113</TT
> for January 13, 1999) or time
(e.g., <TT
CLASS="LITERAL"
>141516</TT
> for 14:15:16).
</P
></LI
><LI
CLASS="STEP"
><P
> If the token starts with a plus (<TT
CLASS="LITERAL"
>+</TT
>) or minus
(<TT
CLASS="LITERAL"
>-</TT
>), then it is either a time zone or a special
field.
</P
></LI
></OL
></LI
><LI
CLASS="STEP"
><P
> If the token is a text string, match up with possible strings.
</P
><OL
CLASS="SUBSTEPS"
TYPE="a"
><LI
CLASS="STEP"
><P
> Do a binary-search table lookup for the token
as either a special string (e.g., <TT
CLASS="LITERAL"
>today</TT
>),
day (e.g., <TT
CLASS="LITERAL"
>Thursday</TT
>),
month (e.g., <TT
CLASS="LITERAL"
>January</TT
>),
or noise word (e.g., <TT
CLASS="LITERAL"
>at</TT
>, <TT
CLASS="LITERAL"
>on</TT
>).
</P
><P
> Set field values and bit mask for fields.
For example, set year, month, day for <TT
CLASS="LITERAL"
>today</TT
>,
and additionally hour, minute, second for <TT
CLASS="LITERAL"
>now</TT
>.
</P
></LI
><LI
CLASS="STEP"
><P
> If not found, do a similar binary-search table lookup to match
the token with a time zone.
</P
></LI
><LI
CLASS="STEP"
><P
> If still not found, throw an error.
</P
></LI
></OL
></LI
><LI
CLASS="STEP"
><P
> When the token is a number or number field:
</P
><OL
CLASS="SUBSTEPS"
TYPE="a"
><LI
CLASS="STEP"
><P
> If there are eight or six digits,
and if no other date fields have been previously read, then interpret
as a <SPAN
CLASS="QUOTE"
>"concatenated date"</SPAN
> (e.g.,
<TT
CLASS="LITERAL"
>19990118</TT
> or <TT
CLASS="LITERAL"
>990118</TT
>).
The interpretation is <TT
CLASS="LITERAL"
>YYYYMMDD</TT
> or <TT
CLASS="LITERAL"
>YYMMDD</TT
>.
</P
></LI
><LI
CLASS="STEP"
><P
> If the token is three digits
and a year has already been read, then interpret as day of year.
</P
></LI
><LI
CLASS="STEP"
><P
> If four or six digits and a year has already been read, then
interpret as a time (<TT
CLASS="LITERAL"
>HHMM</TT
> or <TT
CLASS="LITERAL"
>HHMMSS</TT
>).
</P
></LI
><LI
CLASS="STEP"
><P
> 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).
</P
></LI
><LI
CLASS="STEP"
><P
> Otherwise the date field ordering is assumed to follow the
<TT
CLASS="VARNAME"
>DateStyle</TT
> 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.
</P
></LI
></OL
></LI
><LI
CLASS="STEP"
><P
> 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.)
</P
></LI
><LI
CLASS="STEP"
><P
> 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.
<DIV
CLASS="TIP"
><BLOCKQUOTE
CLASS="TIP"
><P
><B
>Tip: </B
> Gregorian years AD 1-99 may be entered by using 4 digits with leading
zeros (e.g., <TT
CLASS="LITERAL"
>0099</TT
> is AD 99). Previous versions of
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> accepted years with three
digits and with single digits, but as of version 7.0 the rules have
been tightened up to reduce the possibility of ambiguity.
</P
></BLOCKQUOTE
></DIV
>
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