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 parse_datetime function) is not described here.

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

  * combined date and time of day 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 --rfc-3339=ns  # --rfc-3339 is a GNU extension.
     2004-02-29 16:21:42.692722128-08:00
     $ 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.

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

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.

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, timestamps are interpreted using the rules of the default time zone (see Specifying time zone rules).

Combined date and time of day items
===================================

The ISO 8601 date and time of day extended format consists of an ISO 8601 date, a T character separator, and an ISO 8601 time of day.  This format is also recognized if the 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:

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

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.

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.

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 (see 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.

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

If you precede a number with @, it represents an internal timestamp 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 timestamp.

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.

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.  See Specifying the Time Zone with TZ (The GNU C Library).

Authors of parse_datetime
=========================

parse_datetime started life as getdate, as originally implemented by Steven M. Bellovin (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 (rsalz@bbn.com) and Jim Berets (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 get_date to avoid a conflict with the alternative Posix function getdate, and a later rename to parse_datetime.  The Posix function getdate can parse more locale-specific dates using strptime, but relies on an environment variable and external file, and lacks the thread-safety of parse_datetime.

This chapter was originally produced by François Pinard (pinard@iro.umontreal.ca) from the parse_datetime.y source code, and then edited by K. Berry (kb@cs.umb.edu).