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% File src/library/base/man/strptime.Rd
% Part of the R package, http://www.R-project.org
% Copyright 1995-2010 R Core Development Team
% Distributed under GPL 2 or later

\name{strptime}
\alias{format.POSIXct}
\alias{format.POSIXlt}
\alias{strftime}
\alias{strptime}
\alias{as.character.POSIXt}
\title{Date-time Conversion Functions to and from Character}
\description{
  Functions to convert between character representations and objects of
  classes \code{"POSIXlt"} and \code{"POSIXct"} representing calendar
  dates and times.
}
\usage{
\method{format}{POSIXct}(x, format = "", tz = "", usetz = FALSE, \dots)
\method{format}{POSIXlt}(x, format = "", usetz = FALSE, \dots)

\method{as.character}{POSIXt}(x, \dots)

strftime(x, format="", tz = "", usetz = FALSE, \dots)
strptime(x, format, tz = "")
}
\arguments{
  \item{x}{An object to be converted.}
  \item{tz}{A timezone specification to be used for the conversion.
    System-specific (see \code{\link{as.POSIXlt}}), but \code{""} is the
    current time zone, and \code{"GMT"} is UTC.}
  \item{format}{A character string.  The default for the \code{format}
    methods is
    \code{"\%Y-\%m-\%d \%H:\%M:\%S"} if any component has a time
    component which is not midnight, and \code{"\%Y-\%m-\%d"}
    otherwise.  If \code{\link{options}("digits.secs")} is set, up to
    the specified number of digits will be printed for seconds.}
  \item{\dots}{Further arguments to be passed from or to other methods.}
  \item{usetz}{logical.  Should the timezone be appended to the output?
    This is used in printing times, and as a workaround for problems with
    using \code{"\%Z"} on some Linux systems.}
}
\details{
  The \code{format} and \code{as.character} methods and \code{strftime}
  convert objects from the classes \code{"POSIXlt"} and \code{"POSIXct"}
  (not \code{strftime}) to character vectors.
  
  \code{strptime} converts character strings to class \code{"POSIXlt"}:
  its input \code{x} is first coerced to character if necessary.  Each
  string is processed as far as necessary for the format specified: any
  trailing characters are ignored.

  \code{strftime} is a wrapper for \code{format.POSIXlt}, and it and
  \code{format.POSIXct} first converts to class \code{"POSIXlt"} by
  calling \code{\link{as.POSIXlt}}.  Note that only that conversion
  depends on the time zone.

  The usual vector re-cycling rules are applied to \code{x} and
  \code{format} so the answer will be of length that of the longer of the
  vectors.

  Locale-specific conversions to and from character strings are used
  where appropriate and available.  This affects the names of the days
  and months, the AM/PM indicator (if used) and the separators in
  formats such as \code{\%x} and \code{\%X} (via the setting of the
  \code{LC_TIME} locale category).

  The details of the formats are system-specific, but the following are
  defined by the ISO C99 / POSIX standard for \code{strftime} and are
  likely to be widely available.  A \emph{conversion specification} is
  introduced by \code{\%}, usually followed by a single letter or
  \code{O} or \code{E} and then a single letter.
  Any character in the format string not part of a conversion specification
  is interpreted literally (and \code{\%\%} gives \code{\%}).  Widely
  implemented conversion specifications include
  \describe{
    \item{\code{\%a}}{Abbreviated weekday name in the current
      locale. (Also matches full name on input.)}
    \item{\code{\%A}}{Full weekday name in the current locale.  (Also
      matches abbreviated name on input.)}
    \item{\code{\%b}}{Abbreviated month name in the current
      locale. (Also matches full name on input.)}
    \item{\code{\%B}}{Full month name in the current locale.  (Also
      matches abbreviated name on input.)}
    \item{\code{\%c}}{Date and time.   Locale-specific on output,
      \code{"\%a \%b \%e \%H:\%M:\%S \%Y"} on input.}
    \item{\code{\%d}}{Day of the month as decimal number (01--31).}
    \item{\code{\%H}}{Hours as decimal number (00--23).}
    \item{\code{\%I}}{Hours as decimal number (01--12).}
    \item{\code{\%j}}{Day of year as decimal number (001--366).}
    \item{\code{\%m}}{Month as decimal number (01--12).}
    \item{\code{\%M}}{Minute as decimal number (00--59).}
    \item{\code{\%p}}{AM/PM indicator in the locale.  Used in
      conjunction with \code{\%I} and \bold{not} with \code{\%H}.  An
      empty string in some locales.}
    \item{\code{\%S}}{Second as decimal number (00--61), allowing for
      up to two leap-seconds (but POSIX-compliant implementations
      will ignore leap seconds).}
    % see http://en.wikipedia.org/wiki/Week_number#Week_number
    \item{\code{\%U}}{Week of the year as decimal number (00--53) using
      Sunday as the first day 1 of the week (and typically with the
      first Sunday of the year as day 1 of week 1).  The US convention.}
    \item{\code{\%w}}{Weekday as decimal number (0--6, Sunday is 0).}
    \item{\code{\%W}}{Week of the year as decimal number (00--53) using
      Monday as the first day of week (and typically with the
      first Monday of the year as day 1 of week 1). The UK convention.}
    \item{\code{\%x}}{Date.  Locale-specific on output,
      \code{"\%y/\%m/\%d"} on input.}
    \item{\code{\%X}}{Time.  Locale-specific on output,
      \code{"\%H:\%M:\%S"} on input.}
    \item{\code{\%y}}{Year without century (00--99). Values 00 to 68 are
      prefixed by 20 and 69 to 99 by 19 -- that is the behaviour specified
      by the 2004 POSIX standard, but it does also say \sQuote{it is
    expected that in a future version of IEEE Std 1003.1-2001 the default
    century inferred from a 2-digit year will change}.
    }
    \item{\code{\%Y}}{Year with century.}
    \item{\code{\%z}}{Signed offset in hours and minutes from UTC, so
      \code{-0800} is 8 hours behind UTC.}
    \item{\code{\%Z}}{(output only.) Time zone as a character
      string (empty if not available).}
  }
  Where leading zeros are shown they will be used on output but are
  optional on input.
  
  Note that when \code{\%z} or \code{\%Z} is used for output with an
  object with an assigned timezone an attempt is made to use the values
  for that timezone --- but it is not guaranteed to succeed.
  
  Also defined in the current standards but less widely implemented
  (e.g. not for output on Windows) are
  \describe{
    \item{\code{\%C}}{Century (00--99): the integer part of the year
      divided by 100.}
    \item{\code{\%D}}{Date format such as \code{\%m/\%d/\%y}: ISO C99
      says it should be that exact format.}
    \item{\code{\%e}}{Day of the month as decimal number (1--31), with
      a leading space for a single-digit number.}
    \item{\code{\%F}}{Equivalent to \%Y-\%m-\%d (the ISO 8601 date
      format).}
    \item{\code{\%g}}{The last two digits of the week-based year
      (see \code{\%V}).  (Accepted but ignored on input.)}
    \item{\code{\%G}}{The week-based year (see \code{\%V}) as a decimal
      number.  (Accepted but ignored on input.)}
    \item{\code{\%h}}{Equivalent to \code{\%b}.}
    \item{\code{\%k}}{The 24-hour clock time with single digits preceded
      by a blank.}
    \item{\code{\%l}}{The 12-hour clock time with single digits preceded
      by a blank.}
    \item{\code{\%n}}{Newline on output, arbitrary whitespace on input.}
    \item{\code{\%r}}{The 12-hour clock time (using the locale's AM or PM).}
    \item{\code{\%R}}{Equivalent to \code{\%H:\%M}.}
    \item{\code{\%t}}{Tab on output, arbitrary whitespace on input.}
    \item{\code{\%T}}{Equivalent to \code{\%H:\%M:\%S}.}
    \item{\code{\%u}}{Weekday as a decimal number (1--7, Monday is 1).}
    \item{\code{\%V}}{Week of the year as decimal number (00--53) as
      defined in ISO 8601.
      If the week (starting on Monday) containing 1 January has four or
      more days in the new year, then it is considered week 1.  Otherwise, it
      is the last week of the previous year, and the next week is week
      1.  (Accepted but ignored on input.)}
  }
  For output there are also \code{\%O[dHImMUVwWy]} which may emit
  numbers in an alternative locale-dependent format (e.g. roman
  numerals), and \code{\%E[cCyYxX]} which can use an alternative
  \sQuote{era} (e.g. a different religious calendar).  Which of these
  are supported is OS-dependent.  These are accepted for input, but with
  the standard interpretation.

  Specific to \R is \code{\%OSn}, which for output gives the
  seconds to \code{0 <= n <= 6} decimal places (and if \code{\%OS} is
  not followed by a digit, it uses the setting of
  \code{\link{getOption}("digits.secs")}, or if that is unset, \code{n = 3}).
  Further, for \code{strptime} \code{\%OS} will input seconds including
  fractional seconds.  Note that \code{\%S} ignores (and not rounds)
  fractional parts on output.

  The behaviour of other conversion specifications (and even if other
  character sequences commencing with \code{\%} \emph{are} conversion
  specifications) is system-specific.
#ifdef windows
  For output on Windows, a conversion specification is \code{\%}
  optionally followed by \code{#} and then by a single letter.  Any
  conversion specification which is unimplemented is ignored.
#endif
}
\value{
  The \code{format} methods and \code{strftime} return character vectors
  representing the time.  \code{NA} times are returned as \code{NA_character_}.

  \code{strptime} turns character representations into an object of
  class \code{"\link{POSIXlt}"}.  The timezone is used to set the
  \code{isdst} component and to set the \code{"tzone"} attribute if
  \code{tz != ""}.  If the specified time is invalid (in the specified
  timezone) all the components of the result are \code{NA}.
}
\note{
  The default formats follow the rules of the ISO 8601 international
  standard which expresses a day as \code{"2001-02-28"} and a time as
  \code{"14:01:02"} using leading zeroes as here.  The ISO form uses no
  space to separate dates and times.
  
  For \code{strptime} the input string need not specify the date
  completely: it is assumed that unspecified seconds, minutes or hours
  are zero, and an unspecified year, month or day is the current one.
  
  If the timezone specified is invalid on your system, what happens is
  system-specific but it will probably be ignored.

  OS facilities will probably not print years before 1CE (aka 1AD)
  correctly.

  Remember that in most timezones some times do not occur and some occur
  twice because of transitions to/from summer time.  What happens in
  those cases is OS-specific.
}
\references{
  International Organization for Standardization (2004, 2000, 1988, 1997,
  \dots)
  \emph{ISO 8601. Data elements and interchange formats --
    Information interchange -- Representation of dates and times.}
  For links to versions available on-line see (at the time of writing)
  \url{http://www.qsl.net/g1smd/isopdf.htm}; for information on the
  current official version, see
  \url{http://www.iso.org/iso/en/prods-services/popstds/datesandtime.html}.
}
\seealso{
  \link{DateTimeClasses} for details of the date-time classes;
  \code{\link{locales}} to query or set a locale.

  Your system's help pages on \code{strftime} and \code{strptime} to
  see how to specify their formats.
#ifdef unix
  (On some systems \code{strptime} is replaced by corrected code from
  \samp{glibc}, when all the conversion specifications described here
  are supported, but with no alternative number representation nor era
  available in any locale.)
#endif
#ifdef windows
  Windows users will find no help page for \code{strptime}: code based
  on \samp{glibc} is used (with corrections), so all the conversion
  specifications described here are supported, but with no alternative
  number representation nor era available in any locale.
#endif
}
\examples{\donttest{
## locale-specific version of date()
format(Sys.time(), "\%a \%b \%d \%X \%Y \%Z")

## time to sub-second accuracy (if supported by the OS)
format(Sys.time(), "\%H:\%M:\%OS3")

## read in date info in format 'ddmmmyyyy'
## This will give NA(s) in some locales; setting the C locale
## as in the commented lines will overcome this on most systems.
## lct <- Sys.getlocale("LC_TIME"); Sys.setlocale("LC_TIME", "C")
x <- c("1jan1960", "2jan1960", "31mar1960", "30jul1960")
z <- strptime(x, "\%d\%b\%Y")
## Sys.setlocale("LC_TIME", lct)
z

## read in date/time info in format 'm/d/y h:m:s'
dates <- c("02/27/92", "02/27/92", "01/14/92", "02/28/92", "02/01/92")
times <- c("23:03:20", "22:29:56", "01:03:30", "18:21:03", "16:56:26")
x <- paste(dates, times)
strptime(x, "\%m/\%d/\%y \%H:\%M:\%S")

## time with fractional seconds
z <- strptime("20/2/06 11:16:16.683", "\%d/\%m/\%y \%H:\%M:\%OS")
z # prints without fractional seconds
op <- options(digits.secs=3)
z
options(op)

## timezones are not portable, but 'EST5EDT' comes pretty close.
(x <- strptime(c("2006-01-08 10:07:52", "2006-08-07 19:33:02"),
               "\%Y-\%m-\%d \%H:\%M:\%S", tz="EST5EDT"))
attr(x, "tzone")

## An RFC 822 header (Eastern Canada, during DST)
strptime("Tue, 23 Mar 2010 14:36:38 -0400",  "\%a, \%d \%b \%Y \%H:\%M:\%S \%z")
}}
\keyword{utilities}
\keyword{chron}