getdate.texi

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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.  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,
time stamps are interpreted using the rules of the default time zone
(@pxref{Specifying time zone rules}).


@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{2003-07-31 -1 month} might evaluate to 2003-07-01,
because 2003-06-31 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 2003 13:02:39 -0700
$ 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 time
stamp 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 time stamp.

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

Traditional Unix 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 @sc{utc}.  More modern systems use 64-bit counts
of seconds with nanosecond subcounts, and can represent all the times
in the known lifetime of the universe to a resolution of 1 nanosecond.

On most hosts, these counts ignore the presence of leap seconds.
For example, on most hosts @samp{@@915148799} represents 1998-12-31
23:59:59 @sc{utc}, @samp{@@915148800} represents 1999-01-01 00:00:00
@sc{utc}, and there is no way to represent the intervening leap second
1998-12-31 23:59:60 @sc{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 @acronym{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, 2004?'' 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" 2004-10-31 06:30'
Sun Oct 31 01:30:00 EDT 2004
@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{2004-10-31
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 2004, 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{http://www.twinsun.com/tz/tz-link.htm, @samp{tz} database}.
A recent catalog of location names appears in the
@uref{http://twiki.org/cgi-bin/xtra/tzdate, TWiki Date and Time
Gateway}.  A few non-@acronym{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-@acronym{GNU} host that does not support the @samp{tz}
database, you may need to use a @acronym{POSIX} rule instead.  Simple
@acronym{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 get_date
@section Authors of @code{get_date}

@cindex authors of @code{get_date}

@cindex Bellovin, Steven M.
@cindex Salz, Rich
@cindex Berets, Jim
@cindex MacKenzie, David
@cindex Meyering, Jim
@cindex Eggert, Paul
@code{get_date} was 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 @sc{gnu} system were made by David MacKenzie, Jim Meyering,
Paul Eggert and others.

@cindex Pinard, F.
@cindex Berry, K.
This chapter was originally produced by Fran@,{c}ois Pinard
(@email{pinard@@iro.umontreal.ca}) from the @file{getdate.y} source code,
and then edited by K.@: Berry (@email{kb@@cs.umb.edu}).