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@c GNU date syntax documentation

@c Copyright (C) 1994--2006, 2009--2021 Free Software Foundation, Inc.

@c Permission is granted to copy, distribute and/or modify this document
@c under the terms of the GNU Free Documentation License, Version 1.3 or
@c any later version published by the Free Software Foundation; with no
@c Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts.  A
@c copy of the license is at <https://www.gnu.org/licenses/fdl-1.3.en.html>.

@node Date input formats
@chapter Date input formats

@cindex date input formats
@findex parse_datetime

First, a quote:

@quotation
Our units of temporal measurement, from seconds on up to months, are so
complicated, asymmetrical and disjunctive so as to make coherent mental
reckoning in time all but impossible.  Indeed, had some tyrannical god
contrived to enslave our minds to time, to make it all but impossible
for us to escape subjection to sodden routines and unpleasant surprises,
he could hardly have done better than handing down our present system.
It is like a set of trapezoidal building blocks, with no vertical or
horizontal surfaces, like a language in which the simplest thought
demands ornate constructions, useless particles and lengthy
circumlocutions.  Unlike the more successful patterns of language and
science, which enable us to face experience boldly or at least
level-headedly, our system of temporal calculation silently and
persistently encourages our terror of time.

@dots{}  It is as though architects had to measure length in feet, width
in meters and height in ells; as though basic instruction manuals
demanded a knowledge of five different languages.  It is no wonder then
that we often look into our own immediate past or future, last Tuesday
or a week from Sunday, with feelings of helpless confusion.  @dots{}

---Robert Grudin, @cite{Time and the Art of Living}.
@end quotation

This section describes the textual date representations that GNU
programs accept.  These are the strings you, as a user, can supply as
arguments to the various programs.  The C interface (via the
@code{parse_datetime} function) is not described here.

@menu
* General date syntax::          Common rules
* Calendar date items::          21 Jul 2020
* Time of day items::            9:20pm
* Time zone items::              UTC, -0700, +0900, @dots{}
* Combined date and time of day items:: 2020-07-21T20:02:00,000000-0400
* Day of week items::            Monday and others
* Relative items in date strings:: next tuesday, 2 years ago
* Pure numbers in date strings:: 20200721, 1440
* Seconds since the Epoch::      @@1595289600
* Specifying time zone rules::   TZ="America/New_York", TZ="UTC0"
* Authors of parse_datetime::    Bellovin, Eggert, Salz, Berets, et al.
@end menu


@node General date syntax
@section General date syntax

@cindex general date syntax

@cindex items in date strings
A @dfn{date} is a string, possibly empty, containing many items
separated by whitespace.  The whitespace may be omitted when no
ambiguity arises.  The empty string means the beginning of today (i.e.,
midnight).  Order of the items is immaterial.  A date string may contain
many flavors of items:

@itemize @bullet
@item calendar date items
@item time of day items
@item time zone items
@item combined date and time of day items
@item day of the week items
@item relative items
@item pure numbers.
@end itemize

@noindent We describe each of these item types in turn, below.

@cindex numbers, written-out
@cindex ordinal numbers
@findex first @r{in date strings}
@findex next @r{in date strings}
@findex last @r{in date strings}
A few ordinal numbers may be written out in words in some contexts.  This is
most useful for specifying day of the week items or relative items (see
below).  Among the most commonly used ordinal numbers, the word
@samp{last} stands for @math{-1}, @samp{this} stands for 0, and
@samp{first} and @samp{next} both stand for 1.  Because the word
@samp{second} stands for the unit of time there is no way to write the
ordinal number 2, but for convenience @samp{third} stands for 3,
@samp{fourth} for 4, @samp{fifth} for 5,
@samp{sixth} for 6, @samp{seventh} for 7, @samp{eighth} for 8,
@samp{ninth} for 9, @samp{tenth} for 10, @samp{eleventh} for 11 and
@samp{twelfth} for 12.

@cindex months, written-out
When a month is written this way, it is still considered to be written
numerically, instead of being ``spelled in full''; this changes the
allowed strings.

@cindex language, in dates
In the current implementation, only English is supported for words and
abbreviations like @samp{AM}, @samp{DST}, @samp{EST}, @samp{first},
@samp{January}, @samp{Sunday}, @samp{tomorrow}, and @samp{year}.

@cindex language, in dates
@cindex time zone item
The output of the @command{date} command
is not always acceptable as a date string,
not only because of the language problem, but also because there is no
standard meaning for time zone items like @samp{IST}@.  When using
@command{date} to generate a date string intended to be parsed later,
specify a date format that is independent of language and that does not
use time zone items other than @samp{UTC} and @samp{Z}@.  Here are some
ways to do this:

@example
$ LC_ALL=C TZ=UTC0 date
Tue Jul 21 23:00:37 UTC 2020
$ TZ=UTC0 date +'%Y-%m-%d %H:%M:%SZ'
2020-07-21 23:00:37Z
$ date --rfc-3339=ns  # --rfc-3339 is a GNU extension.
2020-07-21 19:00:37.692722128-04:00
$ date --rfc-2822  # a GNU extension
Tue, 21 Jul 2020 19:00:37 -0400
$ date +'%Y-%m-%d %H:%M:%S %z'  # %z is a GNU extension.
2020-07-21 19:00:37 -0400
$ date +'@@%s.%N'  # %s and %N are GNU extensions.
@@1595372437.692722128
@end example

@cindex case, ignored in dates
@cindex comments, in dates
Alphabetic case is completely ignored in dates.  Comments may be introduced
between round parentheses, as long as included parentheses are properly
nested.  Hyphens not followed by a digit are currently ignored.  Leading
zeros on numbers are ignored.

@cindex leap seconds
Invalid dates like @samp{2019-02-29} or times like @samp{24:00} are
rejected.  In the typical case of a host that does not support leap
seconds, a time like @samp{23:59:60} is rejected even if it
corresponds to a valid leap second.


@node Calendar date items
@section Calendar date items

@cindex calendar date item

A @dfn{calendar date item} specifies a day of the year.  It is
specified differently, depending on whether the month is specified
numerically or literally.  All these strings specify the same calendar date:

@example
2020-07-20     # ISO 8601.
20-7-20        # Assume 19xx for 69 through 99,
               # 20xx for 00 through 68 (not recommended).
7/20/2020      # Common U.S. writing.
20 July 2020
20 Jul 2020    # Three-letter abbreviations always allowed.
Jul 20, 2020
20-jul-2020
20jul2020
@end example

The year can also be omitted.  In this case, the last specified year is
used, or the current year if none.  For example:

@example
7/20
jul 20
@end example

Here are the rules.

@cindex ISO 8601 date format
@cindex date format, ISO 8601
For numeric months, the ISO 8601 format
@samp{@var{year}-@var{month}-@var{day}} is allowed, where @var{year} is
any positive number, @var{month} is a number between 01 and 12, and
@var{day} is a number between 01 and 31.  A leading zero must be present
if a number is less than ten.  If @var{year} is 68 or smaller, then 2000
is added to it; otherwise, if @var{year} is less than 100,
then 1900 is added to it.  The construct
@samp{@var{month}/@var{day}/@var{year}}, popular in the United States,
is accepted.  Also @samp{@var{month}/@var{day}}, omitting the year.

@cindex month names in date strings
@cindex abbreviations for months
Literal months may be spelled out in full: @samp{January},
@samp{February}, @samp{March}, @samp{April}, @samp{May}, @samp{June},
@samp{July}, @samp{August}, @samp{September}, @samp{October},
@samp{November} or @samp{December}.  Literal months may be abbreviated
to their first three letters, possibly followed by an abbreviating dot.
It is also permitted to write @samp{Sept} instead of @samp{September}.

When months are written literally, the calendar date may be given as any
of the following:

@example
@var{day} @var{month} @var{year}
@var{day} @var{month}
@var{month} @var{day} @var{year}
@var{day}-@var{month}-@var{year}
@end example

Or, omitting the year:

@example
@var{month} @var{day}
@end example


@node Time of day items
@section Time of day items

@cindex time of day item

A @dfn{time of day item} in date strings specifies the time on a given
day.  Here are some examples, all of which represent the same time:

@example
20:02:00.000000
20:02
8:02pm
20:02-0500      # In EST (U.S. Eastern Standard Time).
@end example

@cindex leap seconds
More generally, the time of day may be given as
@samp{@var{hour}:@var{minute}:@var{second}}, where @var{hour} is
a number between 0 and 23, @var{minute} is a number between 0 and
59, and @var{second} is a number between 0 and 59 possibly followed by
@samp{.} or @samp{,} and a fraction containing one or more digits.
Alternatively,
@samp{:@var{second}} can be omitted, in which case it is taken to
be zero.  On the rare hosts that support leap seconds, @var{second}
may be 60.

@findex am @r{in date strings}
@findex pm @r{in date strings}
@findex midnight @r{in date strings}
@findex noon @r{in date strings}
If the time is followed by @samp{am} or @samp{pm} (or @samp{a.m.}
or @samp{p.m.}), @var{hour} is restricted to run from 1 to 12, and
@samp{:@var{minute}} may be omitted (taken to be zero).  @samp{am}
indicates the first half of the day, @samp{pm} indicates the second
half of the day.  In this notation, 12 is the predecessor of 1:
midnight is @samp{12am} while noon is @samp{12pm}.
(This is the zero-oriented interpretation of @samp{12am} and @samp{12pm},
as opposed to the old tradition derived from Latin
which uses @samp{12m} for noon and @samp{12pm} for midnight.)

@cindex time zone correction
@cindex minutes, time zone correction by
The time may alternatively be followed by a time zone correction,
expressed as @samp{@var{s}@var{hh}@var{mm}}, where @var{s} is @samp{+}
or @samp{-}, @var{hh} is a number of zone hours and @var{mm} is a number
of zone minutes.
The zone minutes term, @var{mm}, may be omitted, in which case
the one- or two-digit correction is interpreted as a number of hours.
You can also separate @var{hh} from @var{mm} with a colon.
When a time zone correction is given this way, it
forces interpretation of the time relative to
Coordinated Universal Time (UTC), overriding any previous
specification for the time zone or the local time zone.  For example,
@samp{+0530} and @samp{+05:30} both stand for the time zone 5.5 hours
ahead of UTC (e.g., India).
This is the best way to
specify a time zone correction by fractional parts of an hour.
The maximum zone correction is 24 hours.

Either @samp{am}/@samp{pm} or a time zone correction may be specified,
but not both.


@node Time zone items
@section Time zone items

@cindex time zone item

A @dfn{time zone item} specifies an international time zone, indicated
by a small set of letters, e.g., @samp{UTC} or @samp{Z}
for Coordinated Universal
Time.  Any included periods are ignored.  By following a
non-daylight-saving time zone by the string @samp{DST} in a separate
word (that is, separated by some white space), the corresponding
daylight saving time zone may be specified.
Alternatively, a non-daylight-saving time zone can be followed by a
time zone correction, to add the two values.  This is normally done
only for @samp{UTC}; for example, @samp{UTC+05:30} is equivalent to
@samp{+05:30}.

Time zone items other than @samp{UTC} and @samp{Z}
are obsolescent and are not recommended, because they
are ambiguous; for example, @samp{EST} has a different meaning in
Australia than in the United States, and @samp{A} has different
meaning as a military time zone than as an obsolescent
RFC 822 time zone.  Instead, it's better to use
unambiguous numeric time zone corrections like @samp{-0500}, as
described in the previous section.

If neither a time zone item nor a time zone correction is supplied,
timestamps are interpreted using the rules of the default time zone
(@pxref{Specifying time zone rules}).


@node Combined date and time of day items
@section Combined date and time of day items

@cindex combined date and time of day item
@cindex ISO 8601 date and time of day format
@cindex date and time of day format, ISO 8601

The ISO 8601 date and time of day extended format consists of an ISO
8601 date, a @samp{T} character separator, and an ISO 8601 time of
day.  This format is also recognized if the @samp{T} is replaced by a
space.

In this format, the time of day should use 24-hour notation.
Fractional seconds are allowed, with either comma or period preceding
the fraction.  ISO 8601 fractional minutes and hours are not
supported.  Typically, hosts support nanosecond timestamp resolution;
excess precision is silently discarded.

Here are some examples:

@example
2012-09-24T20:02:00.052-05:00
2012-12-31T23:59:59,999999999+11:00
1970-01-01 00:00Z
@end example

@node Day of week items
@section Day of week items

@cindex day of week item

The explicit mention of a day of the week will forward the date
(only if necessary) to reach that day of the week in the future.

Days of the week may be spelled out in full: @samp{Sunday},
@samp{Monday}, @samp{Tuesday}, @samp{Wednesday}, @samp{Thursday},
@samp{Friday} or @samp{Saturday}.  Days may be abbreviated to their
first three letters, optionally followed by a period.  The special
abbreviations @samp{Tues} for @samp{Tuesday}, @samp{Wednes} for
@samp{Wednesday} and @samp{Thur} or @samp{Thurs} for @samp{Thursday} are
also allowed.

@findex next @var{day}
@findex last @var{day}
A number may precede a day of the week item to move forward
supplementary weeks.  It is best used in expression like @samp{third
monday}.  In this context, @samp{last @var{day}} or @samp{next
@var{day}} is also acceptable; they move one week before or after
the day that @var{day} by itself would represent.

A comma following a day of the week item is ignored.


@node Relative items in date strings
@section Relative items in date strings

@cindex relative items in date strings
@cindex displacement of dates

@dfn{Relative items} adjust a date (or the current date if none) forward
or backward.  The effects of relative items accumulate.  Here are some
examples:

@example
1 year
1 year ago
3 years
2 days
@end example

@findex year @r{in date strings}
@findex month @r{in date strings}
@findex fortnight @r{in date strings}
@findex week @r{in date strings}
@findex day @r{in date strings}
@findex hour @r{in date strings}
@findex minute @r{in date strings}
The unit of time displacement may be selected by the string @samp{year}
or @samp{month} for moving by whole years or months.  These are fuzzy
units, as years and months are not all of equal duration.  More precise
units are @samp{fortnight} which is worth 14 days, @samp{week} worth 7
days, @samp{day} worth 24 hours, @samp{hour} worth 60 minutes,
@samp{minute} or @samp{min} worth 60 seconds, and @samp{second} or
@samp{sec} worth one second.  An @samp{s} suffix on these units is
accepted and ignored.

@findex ago @r{in date strings}
The unit of time may be preceded by a multiplier, given as an optionally
signed number.  Unsigned numbers are taken as positively signed.  No
number at all implies 1 for a multiplier.  Following a relative item by
the string @samp{ago} is equivalent to preceding the unit by a
multiplier with value @math{-1}.

@findex day @r{in date strings}
@findex tomorrow @r{in date strings}
@findex yesterday @r{in date strings}
The string @samp{tomorrow} is worth one day in the future (equivalent
to @samp{day}), the string @samp{yesterday} is worth
one day in the past (equivalent to @samp{day ago}).

@findex now @r{in date strings}
@findex today @r{in date strings}
@findex this @r{in date strings}
The strings @samp{now} or @samp{today} are relative items corresponding
to zero-valued time displacement, these strings come from the fact
a zero-valued time displacement represents the current time when not
otherwise changed by previous items.  They may be used to stress other
items, like in @samp{12:00 today}.  The string @samp{this} also has
the meaning of a zero-valued time displacement, but is preferred in
date strings like @samp{this thursday}.

When a relative item causes the resulting date to cross a boundary
where the clocks were adjusted, typically for daylight saving time,
the resulting date and time are adjusted accordingly.

The fuzz in units can cause problems with relative items.  For
example, @samp{2020-07-31 -1 month} might evaluate to 2020-07-01,
because 2020-06-31 is an invalid date.  To determine the previous
month more reliably, you can ask for the month before the 15th of the
current month.  For example:

@example
$ date -R
Thu, 31 Jul 2020 13:02:39 -0400
$ date --date='-1 month' +'Last month was %B?'
Last month was July?
$ date --date="$(date +%Y-%m-15) -1 month" +'Last month was %B!'
Last month was June!
@end example

Also, take care when manipulating dates around clock changes such as
daylight saving leaps.  In a few cases these have added or subtracted
as much as 24 hours from the clock, so it is often wise to adopt
universal time by setting the @env{TZ} environment variable to
@samp{UTC0} before embarking on calendrical calculations.

@node Pure numbers in date strings
@section Pure numbers in date strings

@cindex pure numbers in date strings

The precise interpretation of a pure decimal number depends
on the context in the date string.

If the decimal number is of the form @var{yyyy}@var{mm}@var{dd} and no
other calendar date item (@pxref{Calendar date items}) appears before it
in the date string, then @var{yyyy} is read as the year, @var{mm} as the
month number and @var{dd} as the day of the month, for the specified
calendar date.

If the decimal number is of the form @var{hh}@var{mm} and no other time
of day item appears before it in the date string, then @var{hh} is read
as the hour of the day and @var{mm} as the minute of the hour, for the
specified time of day.  @var{mm} can also be omitted.

If both a calendar date and a time of day appear to the left of a number
in the date string, but no relative item, then the number overrides the
year.


@node Seconds since the Epoch
@section Seconds since the Epoch

If you precede a number with @samp{@@}, it represents an internal
timestamp as a count of seconds.  The number can contain an internal
decimal point (either @samp{.} or @samp{,}); any excess precision not
supported by the internal representation is truncated toward minus
infinity.  Such a number cannot be combined with any other date
item, as it specifies a complete timestamp.

@cindex beginning of time, for POSIX
@cindex Epoch, for POSIX
Internally, computer times are represented as a count of seconds since
an Epoch---a well-defined point of time.  On GNU and
POSIX systems, the Epoch is 1970-01-01 00:00:00 UTC, so
@samp{@@0} represents this time, @samp{@@1} represents 1970-01-01
00:00:01 UTC, and so forth.  GNU and most other
POSIX-compliant systems support such times as an extension
to POSIX, using negative counts, so that @samp{@@-1}
represents 1969-12-31 23:59:59 UTC.

Most modern systems count seconds with 64-bit two's-complement integers
of seconds with nanosecond subcounts, which is a range that includes
the known lifetime of the universe with nanosecond resolution.
Some obsolescent systems count seconds with 32-bit two's-complement
integers and can represent times from 1901-12-13 20:45:52 through
2038-01-19 03:14:07 UTC@.  A few systems sport other time ranges.

@cindex leap seconds
On most hosts, these counts ignore the presence of leap seconds.
For example, on most hosts @samp{@@1483228799} represents 2016-12-31
23:59:59 UTC, @samp{@@1483228800} represents 2017-01-01 00:00:00
UTC, and there is no way to represent the intervening leap second
2016-12-31 23:59:60 UTC.

@node Specifying time zone rules
@section Specifying time zone rules

@vindex TZ
Normally, dates are interpreted using the rules of the current time
zone, which in turn are specified by the @env{TZ} environment
variable, or by a system default if @env{TZ} is not set.  To specify a
different set of default time zone rules that apply just to one date,
start the date with a string of the form @samp{TZ="@var{rule}"}.  The
two quote characters (@samp{"}) must be present in the date, and any
quotes or backslashes within @var{rule} must be escaped by a
backslash.

For example, with the GNU @command{date} command you can
answer the question ``What time is it in New York when a Paris clock
shows 6:30am on October 31, 2019?'' by using a date beginning with
@samp{TZ="Europe/Paris"} as shown in the following shell transcript:

@example
$ export TZ="America/New_York"
$ date --date='TZ="Europe/Paris" 2019-10-31 06:30'
Sun Oct 31 01:30:00 EDT 2019
@end example

In this example, the @option{--date} operand begins with its own
@env{TZ} setting, so the rest of that operand is processed according
to @samp{Europe/Paris} rules, treating the string @samp{2019-10-31
06:30} as if it were in Paris.  However, since the output of the
@command{date} command is processed according to the overall time zone
rules, it uses New York time.  (Paris was normally six hours ahead of
New York in 2019, but this example refers to a brief Halloween period
when the gap was five hours.)

A @env{TZ} value is a rule that typically names a location in the
@uref{https://www.iana.org/time-zones, @samp{tz} database}.
A recent catalog of location names appears in the
@uref{https://twiki.org/cgi-bin/xtra/tzdatepick.html, TWiki Date and Time
Gateway}.  A few non-GNU hosts require a colon before a
location name in a @env{TZ} setting, e.g.,
@samp{TZ=":America/New_York"}.

The @samp{tz} database includes a wide variety of locations ranging
from @samp{Arctic/Longyearbyen} to @samp{Antarctica/South_Pole}, but
if you are at sea and have your own private time zone, or if you are
using a non-GNU host that does not support the @samp{tz}
database, you may need to use a POSIX rule instead.  Simple
POSIX rules like @samp{UTC0} specify a time zone without
daylight saving time; other rules can specify simple daylight saving
regimes.  @xref{TZ Variable,, Specifying the Time Zone with @code{TZ},
libc, The GNU C Library}.

@node Authors of parse_datetime
@section Authors of @code{parse_datetime}
@c the anchor keeps the old node name, to try to avoid breaking links
@anchor{Authors of get_date}

@cindex authors of @code{parse_datetime}

@cindex Bellovin, Steven M.
@cindex Salz, Rich
@cindex Berets, Jim
@cindex MacKenzie, David
@cindex Meyering, Jim
@cindex Eggert, Paul
@code{parse_datetime} started life as @code{getdate}, as originally
implemented by Steven M. Bellovin
(@email{smb@@research.att.com}) while at the University of North Carolina
at Chapel Hill.  The code was later tweaked by a couple of people on
Usenet, then completely overhauled by Rich $alz (@email{rsalz@@bbn.com})
and Jim Berets (@email{jberets@@bbn.com}) in August, 1990.  Various
revisions for the GNU system were made by David MacKenzie, Jim Meyering,
Paul Eggert and others, including renaming it to @code{get_date} to
avoid a conflict with the alternative Posix function @code{getdate},
and a later rename to @code{parse_datetime}.  The Posix function
@code{getdate} can parse more locale-specific dates using
@code{strptime}, but relies on an environment variable and external
file, and lacks the thread-safety of @code{parse_datetime}.

@cindex Pinard, F.
@cindex Berry, K.
This chapter was originally produced by Fran@,{c}ois Pinard
(@email{pinard@@iro.umontreal.ca}) from the @file{parse_datetime.y} source code,
and then edited by K. Berry (@email{kb@@cs.umb.edu}).
kc3-lang/gnulib/doc/parse-datetime.texi

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