@c GNU date syntax documentation @c Copyright (C) 1994-2006, 2009-2017 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 @c Texts. A copy of the license is included in the ``GNU Free @c Documentation License'' file as part of this distribution. @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:: 19 Dec 1994. * Time of day items:: 9:20pm. * Time zone items:: EST, PDT, UTC, @dots{} * Combined date and time of day items:: 1972-09-24T20:02:00,000000-0500. * 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 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 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 @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{2005-02-29} 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 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 @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 9/24 sep 24 @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. 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{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 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. 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. @cindex leap seconds 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 UTC, @samp{@@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. @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, 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-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}).