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\name{strptime}
\alias{format.POSIXct}
\alias{format.POSIXlt}
\alias{strftime}
\alias{strptime}
\alias{as.character.POSIXt}
\alias{ISOdatetime}
\alias{ISOdate}
\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="", usetz = FALSE, \dots)
strptime(x, format, tz = "")

ISOdatetime(year, month, day, hour, min, sec, tz = "")
ISOdate(year, month, day, hour = 12, min = 0, sec = 0, tz = "GMT")
}
\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 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 most Linux systems.}
  \item{year, month, day}{numerical values to specify a day.}
  \item{hour, min, sec}{numerical values for a time within a day.
    Fractional seconds are allowed.}
}
\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 an alias for \code{format.POSIXlt}, 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}.

  The details of the formats are system-specific, but the following are
  defined by the ISO C / 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.}
    \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
      conjuction with \code{\%I} and \bold{not} with \code{\%H}.}
    \item{\code{\%S}}{Second as decimal number (00--61), allowing for
      up to two leap-seconds (but POSIX-compliant OSes will ignore leap
      seconds).}
    \item{\code{\%U}}{Week of the year as decimal number (00--53) using
      the first Sunday as day 1 of week 1.}
    \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
      the first Monday as day 1 of week 1.}
    \item{\code{\%x}}{Date, locale-specific.}
    \item{\code{\%X}}{Time, locale-specific.}
    \item{\code{\%y}}{Year without century (00--99). If you use this on
      input, which century you get is system-specific.  So don't!  Often
      values up to 69 (or 68) are prefixed by 20 and 70(or 69) to 99 by 19.}
    \item{\code{\%Y}}{Year with century.}
#ifdef unix
    \item{\code{\%z}}{(output only.) Offset from Greenwich, so
      \code{-0800} is 8 hours west of Greenwich.}
    \item{\code{\%Z}}{(output only.) Time zone as a character
      string (empty if not available).}
#endif
#ifdef windows
    \item{\code{\%z}, \code{\%Z}}{(output only.) Time zone as a character
      string (empty if not available).  For \code{\%z} this differs
      from POSIX.}
#endif
  }
  Where leading zeros are shown they will be used on output but are
  optional on input.
#ifdef windows
  Windows allows the modifier \code{#} in conversion specifications for
  output: \code{\%#c} and \code{\%#x} use longer forms, and for
  \code{dHIjmMSUwWyY} leading zeros are suppressed.
#endif

  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}}{Locale-specific 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}).  (Typically accepted but ignored on input.)}
    \item{\code{\%G}}{The week-based year (see \code{\%V}) as a decimal
      number.  (Typically 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).
      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.  (Typically accepted but ignored on input.)}
  }
  For output (and possibly input) 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.

  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.

  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

  \code{ISOdatetime} and \code{ISOdate} are convenience wrappers for
  \code{strptime}, that differ only in their defaults and that
  \code{ISOdate} sets a timezone.  (For dates without times it would be
  better to use the \code{"\link{Date}"} class.)
}
\value{
  The \code{format} methods and \code{strftime} return character vectors
  representing the time.

  \code{strptime} turns character representations into an object of
  class \code{"\link{POSIXlt}"}.  The timezone is used to set the
  \code{isdst} component and (as from \R 2.5.0) to set the
  \code{"tzone"} attribute if \code{tz != ""}.

  \code{ISOdatetime} and \code{ISOdate} return an object of class
  \code{"\link{POSIXct}"}.
}
\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.

  If the date string does not specify the date completely, the returned
  answer may be system-specific.  The most common behaviour is to assume
  that unspecified seconds, minutes or hours are zero, and a missing
  year, month or day is the current one.  If it specifies a date
  incorrectly, reliable implementations will give an error and the date
  is reported as \code{NA}.  Unfortunately some common implementations
  (such as \samp{glibc}) are unreliable and guess at the intended meaning.

  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, 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{
## 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")
}
\keyword{utilities}
\keyword{chron}