-@node Date input formats
-@chapter Date input formats
-
-@cindex date input formats
-@findex getdate
-
-@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 @sc{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{getdate} function) is not described here.
-
-@cindex beginning of time, for Unix
-@cindex epoch, for Unix
-Although the date syntax here can represent any possible time since zero
-A.D., computer integers are not big enough for such a (comparatively)
-long time. The earliest date semantically allowed on Unix systems is
-midnight, 1 January 1970 UCT.
-
-@menu
-* General date syntax:: Common rules.
-* Calendar date item:: 19 Dec 1994.
-* Time of day item:: 9:20pm.
-* Time zone item:: @sc{est}, @sc{gmt}, @sc{utc}, ...
-* Day of week item:: Monday and others.
-* Relative item in date strings:: next tuesday, 2 years ago.
-* Pure numbers in date strings:: 19931219, 1440.
-* Authors of getdate:: Bellovin, Berets, Eggert, Salz, 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 white space. The white space 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 the day items
-@item time zone 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 numbers may be written out in words in most contexts. This is
-most useful for specifying day of the week items or relative items (see
-below). Here is the list: @samp{first} for 1, @samp{next} for 2,
-@samp{third} 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. Also, @samp{last} means exactly @math{-1}.
-
-@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 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.
-
-
-@node Calendar date item
-@section Calendar date item
-
-@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
-1970-09-17 # ISO 8601.
-70-9-17 # This century assumed by default.
-70-09-17 # Leading zeros are ignored.
-9/17/72 # Common U.S. writing.
-24 September 1972
-24 Sept 72 # September has a special abbreviation.
-24 Sep 72 # Three-letter abbreviations always allowed.
-Sep 24, 1972
-24-sep-72
-24sep72
-@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
-9/17
-sep 17
-@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 less than 100, then 1900
-is added to it to force a date in this century. 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 item
-@section Time of day item
-
-@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
-20:02:0
-20:02
-8:02pm
-20:02-0500 # In @sc{est} (U.S. Eastern Standard Time).
-@end example
-
-More generally, the time of the 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. Alternatively,
-@samp{:@var{second}} can be omitted, in which case it is taken to
-be zero.
-
-@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 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. When a time zone correction is given this way, it
-forces interpretation of the time in @sc{utc}, overriding any previous
-specification for the time zone or the local time zone. The @var{minute}
-part of the time of the day may not be elided when a time zone correction
-is used.
-
-Either @samp{am}/@samp{pm} or a time zone correction may be specified,
-but not both.
-
-
-@node Time zone item
-@section Time zone item
-
-@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} for Coordinated Universal
-Time. Any included period is 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.
-
-Time zone items 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. Instead, it's better to use
-unambiguous numeric time zone corrections like @samp{-0500}, as
-described in the previous section.
-
-@node Day of week item
-@section Day of week item
-
-@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 item in date strings
-@section Relative item 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 change 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}.
-
-
-@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 item}) 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 the 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 Authors of getdate
-@section Authors of @code{getdate}
-
-@cindex authors of @code{getdate}
-
-@cindex Bellovin, Steven M.
-@cindex Berets, Jim
-@cindex Eggert, Paul
-@cindex MacKenzie, David
-@cindex Meyering, Jim
-@cindex Salz, Rich
-@code{getdate} was originally implemented by Steven M. Bellovin
-(@samp{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 (@samp{rsalz@@bbn.com})
-and Jim Berets (@samp{jberets@@bbn.com}) in August, 1990. Various
-revisions for the @sc{gnu} system were made by David MacKenzie, Jim Meyering,
-and others. The code was rewritten again in August, 1999 by Paul Eggert,
-to improve its support for daylight saving time.
-
-@cindex Pinard, F.
-@cindex Berry, K.
-This chapter was originally produced by Fran@,{c}ois Pinard
-(@samp{pinard@@iro.umontreal.ca}) from the @file{getdate.y} source code,
-and then edited by K.@: Berry (@samp{kb@@cs.umb.edu}).