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File: coreutils.info,  Node: date invocation,  Next: arch invocation,  Up: System context

21.1 `date': Print or set system date and time
==============================================

Synopses:

     date [OPTION]... [+FORMAT]
     date [-u|--utc|--universal] [ MMDDhhmm[[CC]YY][.ss] ]

   Invoking `date' with no FORMAT argument is equivalent to invoking it
with a default format that depends on the `LC_TIME' locale category.
In the default C locale, this format is `'+%a %b %e %H:%M:%S %Z %Y'',
so the output looks like `Thu Mar  3 13:47:51 PST 2005'.

   Normally, `date' uses the time zone rules indicated by the `TZ'
environment variable, or the system default rules if `TZ' is not set.
*Note Specifying the Time Zone with `TZ': (libc)TZ Variable.

   If given an argument that starts with a `+', `date' prints the
current date and time (or the date and time specified by the `--date'
option, see below) in the format defined by that argument, which is
similar to that of the `strftime' function.  Except for conversion
specifiers, which start with `%', characters in the format string are
printed unchanged.  The conversion specifiers are described below.

   An exit status of zero indicates success, and a nonzero value
indicates failure.

* Menu:

* Time conversion specifiers::     %[HIklMNpPrRsSTXzZ]
* Date conversion specifiers::     %[aAbBcCdDeFgGhjmuUVwWxyY]
* Literal conversion specifiers::  %[%nt]
* Padding and other flags::        Pad with zeros, spaces, etc.
* Setting the time::               Changing the system clock.
* Options for date::               Instead of the current time.
* Date input formats::             Specifying date strings.
* Examples of date::               Examples.

File: coreutils.info,  Node: Time conversion specifiers,  Next: Date conversion specifiers,  Up: date invocation

21.1.1 Time conversion specifiers
---------------------------------

`date' conversion specifiers related to times.

`%H'
     hour (`00'...`23')

`%I'
     hour (`01'...`12')

`%k'
     hour (` 0'...`23').  This is a GNU extension.

`%l'
     hour (` 1'...`12').  This is a GNU extension.

`%M'
     minute (`00'...`59')

`%N'
     nanoseconds (`000000000'...`999999999').  This is a GNU extension.

`%p'
     locale's equivalent of either `AM' or `PM'; blank in many locales.
     Noon is treated as `PM' and midnight as `AM'.

`%P'
     like `%p', except lower case.  This is a GNU extension.

`%r'
     locale's 12-hour clock time (e.g., `11:11:04 PM')

`%R'
     24-hour hour and minute.  Same as `%H:%M'.  This is a GNU
     extension.

`%s'
     seconds since the epoch, i.e., since 1970-01-01 00:00:00 UTC.
     Leap seconds are not counted unless leap second support is
     available.  *Note %s-examples::, for examples.  This is a GNU
     extension.

`%S'
     second (`00'...`60').  This may be `60' if leap seconds are
     supported.

`%T'
     24-hour hour, minute, and second.  Same as `%H:%M:%S'.

`%X'
     locale's time representation (e.g., `23:13:48')

`%z'
     RFC 2822/ISO 8601 style numeric time zone (e.g., `-0600' or
     `+0530'), or nothing if no time zone is determinable.  This value
     reflects the numeric time zone appropriate for the current time,
     using the time zone rules specified by the `TZ' environment
     variable.  The time (and optionally, the time zone rules) can be
     overridden by the `--date' option.  This is a GNU extension.

`%:z'
     RFC 3339/ISO 8601 style numeric time zone with `:' (e.g., `-06:00'
     or `+05:30'), or nothing if no time zone is determinable.  This is
     a GNU extension.

`%::z'
     Numeric time zone to the nearest second with `:' (e.g.,
     `-06:00:00' or `+05:30:00'), or nothing if no time zone is
     determinable.  This is a GNU extension.

`%:::z'
     Numeric time zone with `:' using the minimum necessary precision
     (e.g., `-06', `+05:30', or `-04:56:02'), or nothing if no time
     zone is determinable.  This is a GNU extension.

`%Z'
     alphabetic time zone abbreviation (e.g., `EDT'), or nothing if no
     time zone is determinable.  See `%z' for how it is determined.

File: coreutils.info,  Node: Date conversion specifiers,  Next: Literal conversion specifiers,  Prev: Time conversion specifiers,  Up: date invocation

21.1.2 Date conversion specifiers
---------------------------------

`date' conversion specifiers related to dates.

`%a'
     locale's abbreviated weekday name (e.g., `Sun')

`%A'
     locale's full weekday name, variable length (e.g., `Sunday')

`%b'
     locale's abbreviated month name (e.g., `Jan')

`%B'
     locale's full month name, variable length (e.g., `January')

`%c'
     locale's date and time (e.g., `Thu Mar  3 23:05:25 2005')

`%C'
     century.  This is like `%Y', except the last two digits are
     omitted.  For example, it is `20' if `%Y' is `2000', and is `-0'
     if `%Y' is `-001'.  It is normally at least two characters, but it
     may be more.

`%d'
     day of month (e.g., `01')

`%D'
     date; same as `%m/%d/%y'

`%e'
     day of month, space padded; same as `%_d'

`%F'
     full date in ISO 8601 format; same as `%Y-%m-%d'.  This is a good
     choice for a date format, as it is standard and is easy to sort in
     the usual case where years are in the range 0000...9999.  This is
     a GNU extension.

`%g'
     year corresponding to the ISO week number, but without the century
     (range `00' through `99').  This has the same format and value as
     `%y', except that if the ISO week number (see `%V') belongs to the
     previous or next year, that year is used instead.  This is a GNU
     extension.

`%G'
     year corresponding to the ISO week number.  This has the same
     format and value as `%Y', except that if the ISO week number (see
     `%V') belongs to the previous or next year, that year is used
     instead.  It is normally useful only if `%V' is also used; for
     example, the format `%G-%m-%d' is probably a mistake, since it
     combines the ISO week number year with the conventional month and
     day.  This is a GNU extension.

`%h'
     same as `%b'

`%j'
     day of year (`001'...`366')

`%m'
     month (`01'...`12')

`%u'
     day of week (`1'...`7') with `1' corresponding to Monday

`%U'
     week number of year, with Sunday as the first day of the week
     (`00'...`53').  Days in a new year preceding the first Sunday are
     in week zero.

`%V'
     ISO week number, that is, the week number of year, with Monday as
     the first day of the week (`01'...`53').  If the week containing
     January 1 has four or more days in the new year, then it is
     considered week 1; otherwise, it is week 53 of the previous year,
     and the next week is week 1.  (See the ISO 8601 standard.)

`%w'
     day of week (`0'...`6') with 0 corresponding to Sunday

`%W'
     week number of year, with Monday as first day of week
     (`00'...`53').  Days in a new year preceding the first Monday are
     in week zero.

`%x'
     locale's date representation (e.g., `12/31/99')

`%y'
     last two digits of year (`00'...`99')

`%Y'
     year.  This is normally at least four characters, but it may be
     more.  Year `0000' precedes year `0001', and year `-001' precedes
     year `0000'.

File: coreutils.info,  Node: Literal conversion specifiers,  Next: Padding and other flags,  Prev: Date conversion specifiers,  Up: date invocation

21.1.3 Literal conversion specifiers
------------------------------------

`date' conversion specifiers that produce literal strings.

`%%'
     a literal %

`%n'
     a newline

`%t'
     a horizontal tab

File: coreutils.info,  Node: Padding and other flags,  Next: Setting the time,  Prev: Literal conversion specifiers,  Up: date invocation

21.1.4 Padding and other flags
------------------------------

Unless otherwise specified, `date' normally pads numeric fields with
zeros, so that, for example, numeric months are always output as two
digits.  Seconds since the epoch are not padded, though, since there is
no natural width for them.

   As a GNU extension, `date' recognizes any of the following optional
flags after the `%':

`-'
     (hyphen) Do not pad the field; useful if the output is intended for
     human consumption.

`_'
     (underscore) Pad with spaces; useful if you need a fixed number of
     characters in the output, but zeros are too distracting.

`0'
     (zero) Pad with zeros even if the conversion specifier would
     normally pad with spaces.

`^'
     Use upper case characters if possible.

`#'
     Use opposite case characters if possible.  A field that is
     normally upper case becomes lower case, and vice versa.

Here are some examples of padding:

     date +%d/%m -d "Feb 1"
     => 01/02
     date +%-d/%-m -d "Feb 1"
     => 1/2
     date +%_d/%_m -d "Feb 1"
     =>  1/ 2

   As a GNU extension, you can specify the field width (after any flag,
if present) as a decimal number.  If the natural size of the output of
the field has less than the specified number of characters, the result
is written right adjusted and padded to the given size.  For example,
`%9B' prints the right adjusted month name in a field of width 9.

   An optional modifier can follow the optional flag and width
specification.  The modifiers are:

`E'
     Use the locale's alternate representation for date and time.  This
     modifier applies to the `%c', `%C', `%x', `%X', `%y' and `%Y'
     conversion specifiers.  In a Japanese locale, for example, `%Ex'
     might yield a date format based on the Japanese Emperors' reigns.

`O'
     Use the locale's alternate numeric symbols for numbers.  This
     modifier applies only to numeric conversion specifiers.

   If the format supports the modifier but no alternate representation
is available, it is ignored.

File: coreutils.info,  Node: Setting the time,  Next: Options for date,  Prev: Padding and other flags,  Up: date invocation

21.1.5 Setting the time
-----------------------

If given an argument that does not start with `+', `date' sets the
system clock to the date and time specified by that argument (as
described below).  You must have appropriate privileges to set the
system clock.  The `--date' and `--set' options may not be used with
such an argument.  The `--universal' option may be used with such an
argument to indicate that the specified date and time are relative to
Coordinated Universal Time rather than to the local time zone.

   The argument must consist entirely of digits, which have the
following meaning:

`MM'
     month

`DD'
     day within month

`hh'
     hour

`mm'
     minute

`CC'
     first two digits of year (optional)

`YY'
     last two digits of year (optional)

`ss'
     second (optional)

   The `--set' option also sets the system clock; see the next section.

File: coreutils.info,  Node: Options for date,  Prev: Setting the time,  Up: date invocation

21.1.6 Options for `date'
-------------------------

The program accepts the following options.  Also see *note Common
options::.

`-d DATESTR'
`--date=DATESTR'
     Display the date and time specified in DATESTR instead of the
     current date and time.  DATESTR can be in almost any common
     format.  It can contain month names, time zones, `am' and `pm',
     `yesterday', etc.  For example, `--date="2004-02-27
     14:19:13.489392193 +0530"' specifies the instant of time that is
     489,392,193 nanoseconds after February 27, 2004 at 2:19:13 PM in a
     time zone that is 5 hours and 30 minutes east of UTC.
     Note: input currently must be in locale independent format. E.g.,
     the LC_TIME=C below is needed to print back the correct date in
     many locales:
          date -d "$(LC_TIME=C date)"
     *Note Date input formats::.

`-f DATEFILE'
`--file=DATEFILE'
     Parse each line in DATEFILE as with `-d' and display the resulting
     date and time.  If DATEFILE is `-', use standard input.  This is
     useful when you have many dates to process, because the system
     overhead of starting up the `date' executable many times can be
     considerable.

`-r FILE'
`--reference=FILE'
     Display the date and time of the last modification of FILE,
     instead of the current date and time.

`-R'
`--rfc-822'
`--rfc-2822'
     Display the date and time using the format `%a, %d %b %Y %H:%M:%S
     %z', evaluated in the C locale so abbreviations are always in
     English.  For example:

          Fri, 09 Sep 2005 13:51:39 -0700

     This format conforms to Internet RFCs 2822
     (ftp://ftp.rfc-editor.org/in-notes/rfc2822.txt) and 822
     (ftp://ftp.rfc-editor.org/in-notes/rfc822.txt), the current and
     previous standards for Internet email.

`--rfc-3339=TIMESPEC'
     Display the date using a format specified by Internet RFC 3339
     (ftp://ftp.rfc-editor.org/in-notes/rfc3339.txt).  This is a subset
     of the ISO 8601 format, except that it also permits applications
     to use a space rather than a `T' to separate dates from times.
     Unlike the other standard formats, RFC 3339 format is always
     suitable as input for the `--date' (`-d') and `--file' (`-f')
     options, regardless of the current locale.

     The argument TIMESPEC specifies how much of the time to include.
     It can be one of the following:

    `date'
          Print just the full-date, e.g., `2005-09-14'.  This is
          equivalent to the format `%Y-%m-%d'.

    `seconds'
          Print the full-date and full-time separated by a space, e.g.,
          `2005-09-14 00:56:06+05:30'.  The output ends with a numeric
          time-offset; here the `+05:30' means that local time is five
          hours and thirty minutes east of UTC.  This is equivalent to
          the format `%Y-%m-%d %H:%M:%S%:z'.

    `ns'
          Like `seconds', but also print nanoseconds, e.g., `2005-09-14
          00:56:06.998458565+05:30'.  This is equivalent to the format
          `%Y-%m-%d %H:%M:%S.%N%:z'.


`-s DATESTR'
`--set=DATESTR'
     Set the date and time to DATESTR.  See `-d' above.

`-u'
`--utc'
`--universal'
     Use Coordinated Universal Time (UTC) by operating as if the `TZ'
     environment variable were set to the string `UTC0'.  Coordinated
     Universal Time is often called "Greenwich Mean Time" (GMT) for
     historical reasons.

File: coreutils.info,  Node: Date input formats,  Next: Opening the software toolbox,  Prev: File permissions,  Up: Top

28 Date input formats
*********************

First, a quote:

     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.

     ...  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.  ...

     -- Robert Grudin, `Time and the Art of Living'.

   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 `get_date'
function) is not described here.

* Menu:

* General date syntax::            Common rules.
* Calendar date items::            19 Dec 1994.
* Time of day items::              9:20pm.
* Time zone items::                EST, PDT, GMT.
* Day of week items::              Monday and others.
* Relative items in date strings:: next tuesday, 2 years ago.
* Pure numbers in date strings::   19931219, 1440.
* Seconds since the Epoch::        @1078100502.
* Specifying time zone rules::     TZ="America/New_York", TZ="UTC0".
* Authors of get_date::            Bellovin, Eggert, Salz, Berets, et al.

File: coreutils.info,  Node: General date syntax,  Next: Calendar date items,  Up: Date input formats

28.1 General date syntax
========================

A "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:

   * calendar date items

   * time of day items

   * time zone items

   * day of the week items

   * relative items

   * pure numbers.

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

   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 `last' stands for -1, `this' stands for 0, and `first' and `next'
both stand for 1.  Because the word `second' stands for the unit of
time there is no way to write the ordinal number 2, but for convenience
`third' stands for 3, `fourth' for 4, `fifth' for 5, `sixth' for 6,
`seventh' for 7, `eighth' for 8, `ninth' for 9, `tenth' for 10,
`eleventh' for 11 and `twelfth' for 12.

   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.

   In the current implementation, only English is supported for words
and abbreviations like `AM', `DST', `EST', `first', `January',
`Sunday', `tomorrow', and `year'.

   The output of the `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 `IST'.  When using
`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 `UTC' and `Z'.  Here are some ways to do this:

     $ LC_ALL=C TZ=UTC0 date
     Mon Mar  1 00:21:42 UTC 2004
     $ TZ=UTC0 date +'%Y-%m-%d %H:%M:%SZ'
     2004-03-01 00:21:42Z
     $ date --iso-8601=ns | tr T ' '  # --iso-8601 is a GNU extension.
     2004-02-29 16:21:42,692722128-0800
     $ date --rfc-2822  # a GNU extension
     Sun, 29 Feb 2004 16:21:42 -0800
     $ date +'%Y-%m-%d %H:%M:%S %z'  # %z is a GNU extension.
     2004-02-29 16:21:42 -0800
     $ date +'@%s.%N'  # %s and %N are GNU extensions.
     @1078100502.692722128

   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.

   Invalid dates like `2005-02-29' or times like `24:00' are rejected.
In the typical case of a host that does not support leap seconds, a
time like `23:59:60' is rejected even if it corresponds to a valid leap
second.

File: coreutils.info,  Node: Calendar date items,  Next: Time of day items,  Prev: General date syntax,  Up: Date input formats

28.2 Calendar date items
========================

A "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:

     1972-09-24     # ISO 8601.
     72-9-24        # Assume 19xx for 69 through 99,
                    # 20xx for 00 through 68.
     72-09-24       # Leading zeros are ignored.
     9/24/72        # 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

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

     9/24
     sep 24

   Here are the rules.

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

   Literal months may be spelled out in full: `January', `February',
`March', `April', `May', `June', `July', `August', `September',
`October', `November' or `December'.  Literal months may be abbreviated
to their first three letters, possibly followed by an abbreviating dot.
It is also permitted to write `Sept' instead of `September'.

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

     DAY MONTH YEAR
     DAY MONTH
     MONTH DAY YEAR
     DAY-MONTH-YEAR

   Or, omitting the year:

     MONTH DAY

File: coreutils.info,  Node: Time of day items,  Next: Time zone items,  Prev: Calendar date items,  Up: Date input formats

28.3 Time of day items
======================

A "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:

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

   More generally, the time of day may be given as
`HOUR:MINUTE:SECOND', where HOUR is a number between 0 and 23, MINUTE
is a number between 0 and 59, and SECOND is a number between 0 and 59
possibly followed by `.' or `,' and a fraction containing one or more
digits.  Alternatively, `:SECOND' can be omitted, in which case it is
taken to be zero.  On the rare hosts that support leap seconds, SECOND
may be 60.

   If the time is followed by `am' or `pm' (or `a.m.' or `p.m.'), HOUR
is restricted to run from 1 to 12, and `:MINUTE' may be omitted (taken
to be zero).  `am' indicates the first half of the day, `pm' indicates
the second half of the day.  In this notation, 12 is the predecessor of
1: midnight is `12am' while noon is `12pm'.  (This is the zero-oriented
interpretation of `12am' and `12pm', as opposed to the old tradition
derived from Latin which uses `12m' for noon and `12pm' for midnight.)

   The time may alternatively be followed by a time zone correction,
expressed as `SHHMM', where S is `+' or `-', HH is a number of zone
hours and MM is a number of zone minutes.  The zone minutes term, MM,
may be omitted, in which case the one- or two-digit correction is
interpreted as a number of hours.  You can also separate HH from 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, `+0530' and `+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 `am'/`pm' or a time zone correction may be specified, but not
both.

File: coreutils.info,  Node: Time zone items,  Next: Day of week items,  Prev: Time of day items,  Up: Date input formats

28.4 Time zone items
====================

A "time zone item" specifies an international time zone, indicated by a
small set of letters, e.g., `UTC' or `Z' for Coordinated Universal
Time.  Any included periods are ignored.  By following a
non-daylight-saving time zone by the string `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
`UTC'; for example, `UTC+05:30' is equivalent to `+05:30'.

   Time zone items other than `UTC' and `Z' are obsolescent and are not
recommended, because they are ambiguous; for example, `EST' has a
different meaning in Australia than in the United States.  Instead,
it's better to use unambiguous numeric time zone corrections like
`-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
(*note Specifying time zone rules::).

File: coreutils.info,  Node: Day of week items,  Next: Relative items in date strings,  Prev: Time zone items,  Up: Date input formats

28.5 Day of week items
======================

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: `Sunday', `Monday',
`Tuesday', `Wednesday', `Thursday', `Friday' or `Saturday'.  Days may
be abbreviated to their first three letters, optionally followed by a
period.  The special abbreviations `Tues' for `Tuesday', `Wednes' for
`Wednesday' and `Thur' or `Thurs' for `Thursday' are also allowed.

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

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

File: coreutils.info,  Node: Relative items in date strings,  Next: Pure numbers in date strings,  Prev: Day of week items,  Up: Date input formats

28.6 Relative items in date strings
===================================

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

     1 year
     1 year ago
     3 years
     2 days

   The unit of time displacement may be selected by the string `year'
or `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 `fortnight' which is worth 14 days, `week' worth 7 days, `day'
worth 24 hours, `hour' worth 60 minutes, `minute' or `min' worth 60
seconds, and `second' or `sec' worth one second.  An `s' suffix on
these units is accepted and ignored.

   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 `ago' is equivalent to preceding the unit
by a multiplier with value -1.

   The string `tomorrow' is worth one day in the future (equivalent to
`day'), the string `yesterday' is worth one day in the past (equivalent
to `day ago').

   The strings `now' or `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 `12:00 today'.  The string `this' also has the meaning
of a zero-valued time displacement, but is preferred in date strings
like `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, `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:

     $ 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!

   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 `TZ' environment variable to `UTC0'
before embarking on calendrical calculations.

File: coreutils.info,  Node: Pure numbers in date strings,  Next: Seconds since the Epoch,  Prev: Relative items in date strings,  Up: Date input formats

28.7 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 YYYYMMDD and no other calendar
date item (*note Calendar date items::) appears before it in the date
string, then YYYY is read as the year, MM as the month number and DD as
the day of the month, for the specified calendar date.

   If the decimal number is of the form HHMM and no other time of day
item appears before it in the date string, then HH is read as the hour
of the day and MM as the minute of the hour, for the specified time of
day.  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.

File: coreutils.info,  Node: Seconds since the Epoch,  Next: Specifying time zone rules,  Prev: Pure numbers in date strings,  Up: Date input formats

28.8 Seconds since the Epoch
============================

If you precede a number with `@', it represents an internal time stamp
as a count of seconds.  The number can contain an internal decimal
point (either `.' or `,'); 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.

   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 `@0' represents this
time, `@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 `@-1' represents 1969-12-31
23:59:59 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 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 `@915148799' represents 1998-12-31 23:59:59
UTC, `@915148800' represents 1999-01-01 00:00:00 UTC, and there is no
way to represent the intervening leap second 1998-12-31 23:59:60 UTC.

File: coreutils.info,  Node: Specifying time zone rules,  Next: Authors of get_date,  Prev: Seconds since the Epoch,  Up: Date input formats

28.9 Specifying time zone rules
===============================

Normally, dates are interpreted using the rules of the current time
zone, which in turn are specified by the `TZ' environment variable, or
by a system default if `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 `TZ="RULE"'.  The two quote characters (`"')
must be present in the date, and any quotes or backslashes within RULE
must be escaped by a backslash.

   For example, with the GNU `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 `TZ="Europe/Paris"' as shown
in the following shell transcript:

     $ export TZ="America/New_York"
     $ date --date='TZ="Europe/Paris" 2004-10-31 06:30'
     Sun Oct 31 01:30:00 EDT 2004

   In this example, the `--date' operand begins with its own `TZ'
setting, so the rest of that operand is processed according to
`Europe/Paris' rules, treating the string `2004-10-31 06:30' as if it
were in Paris.  However, since the output of the `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 `TZ' value is a rule that typically names a location in the `tz'
database (http://www.twinsun.com/tz/tz-link.htm).  A recent catalog of
location names appears in the TWiki Date and Time Gateway
(http://twiki.org/cgi-bin/xtra/tzdate).  A few non-GNU hosts require a
colon before a location name in a `TZ' setting, e.g.,
`TZ=":America/New_York"'.

   The `tz' database includes a wide variety of locations ranging from
`Arctic/Longyearbyen' to `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 `tz' database, you may need to use a POSIX
rule instead.  Simple POSIX rules like `UTC0' specify a time zone
without daylight saving time; other rules can specify simple daylight
saving regimes.  *Note Specifying the Time Zone with `TZ': (libc)TZ
Variable.

File: coreutils.info,  Node: Authors of get_date,  Prev: Specifying time zone rules,  Up: Date input formats

28.10 Authors of `get_date'
===========================

`get_date' was originally implemented by Steven M. Bellovin
(<smbATresearch.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 (<rsalzATbbn.com>) and
Jim Berets (<jberetsATbbn.com>) in August, 1990.  Various revisions for
the GNU system were made by David MacKenzie, Jim Meyering, Paul Eggert
and others.

   This chapter was originally produced by Franc,ois Pinard
(<pinardATiro.ca>) from the `getdate.y' source code, and then
edited by K. Berry (<kbATcs.edu>).

File: coreutils.info,  Node: Examples of date,  Up: date invocation

21.1.7 Examples of `date'
-------------------------

Here are a few examples.  Also see the documentation for the `-d'
option in the previous section.

   * To print the date of the day before yesterday:

          date --date='2 days ago'

   * To print the date of the day three months and one day hence:

          date --date='3 months 1 day'

   * To print the day of year of Christmas in the current year:

          date --date='25 Dec' +%j

   * To print the current full month name and the day of the month:

          date '+%B %d'

     But this may not be what you want because for the first nine days
     of the month, the `%d' expands to a zero-padded two-digit field,
     for example `date -d 1may '+%B %d'' will print `May 01'.

   * To print a date without the leading zero for one-digit days of the
     month, you can use the (GNU extension) `-' flag to suppress the
     padding altogether:

          date -d 1may '+%B %-d

   * To print the current date and time in the format required by many
     non-GNU versions of `date' when setting the system clock:

          date +%m%d%H%M%Y.%S

   * To set the system clock forward by two minutes:

          date --set='+2 minutes'

   * To print the date in RFC 2822 format, use `date --rfc-2822'.  Here
     is some example output:

          Fri, 09 Sep 2005 13:51:39 -0700

   * To convert a date string to the number of seconds since the epoch
     (which is 1970-01-01 00:00:00 UTC), use the `--date' option with
     the `%s' format.  That can be useful in sorting and/or graphing
     and/or comparing data by date.  The following command outputs the
     number of the seconds since the epoch for the time two minutes
     after the epoch:

          date --date='1970-01-01 00:02:00 +0000' +%s
          120

     If you do not specify time zone information in the date string,
     `date' uses your computer's idea of the time zone when
     interpreting the string.  For example, if your computer's time
     zone is that of Cambridge, Massachusetts, which was then 5 hours
     (i.e., 18,000 seconds) behind UTC:

          # local time zone used
          date --date='1970-01-01 00:02:00' +%s
          18120

   * If you're sorting or graphing dated data, your raw date values may
     be represented as seconds since the epoch.  But few people can
     look at the date `946684800' and casually note "Oh, that's the
     first second of the year 2000 in Greenwich, England."

          date --date='2000-01-01 UTC' +%s
          946684800

     An alternative is to use the `--utc' (`-u') option.  Then you may
     omit `UTC' from the date string.  Although this produces the same
     result for `%s' and many other format sequences, with a time zone
     offset different from zero, it would give a different result for
     zone-dependent formats like `%z'.

          date -u --date=2000-01-01 +%s
          946684800

     To convert such an unwieldy number of seconds back to a more
     readable form, use a command like this:

          # local time zone used
          date -d '1970-01-01 UTC 946684800 seconds' +"%Y-%m-%d %T %z"
          1999-12-31 19:00:00 -0500

     Or if you do not mind depending on the `@' feature present since
     coreutils 5.3.0, you could shorten this to:

          date -d @946684800 +"%F %T %z"
          1999-12-31 19:00:00 -0500

     Often it is better to output UTC-relative date and time:

          date -u -d '1970-01-01 946684800 seconds' +"%Y-%m-%d %T %z"
          2000-01-01 00:00:00 +0000