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  They conventionally have the opposite sign from time-zone
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  They conventionally have the opposite sign from time-zone
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  specifications (see \code{\link{Sys.timezone}}): positive values are
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  specifications (see \code{\link{Sys.timezone}}): positive values are
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  East of the meridian.  Although there have been time zones with
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  East of the meridian.  Although there have been time zones with
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  offsets like 00:09:21 (Paris in 1900), and 00:44:30 (Liberia until
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  offsets like 00:09:21 (Paris in 1900), and 00:44:30 (Liberia until
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  1972), offsets are usually treated as whole numbers of minutes, and
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  1972), offsets are usually treated as whole numbers of minutes, and
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  are most often seen in RFC 822 email headers in forms like
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  are most often seen in RFC 5322 email headers in forms like
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  \code{-0800} (e.g., used on the Pacific coast of the US in winter).
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  \code{-0800} (e.g., used on the Pacific coast of the USA in winter).
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  Format \code{\%z} can be used for input or output: it is a character
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  Format \code{\%z} can be used for input or output: it is a character
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  string, conventionally plus or minus followed by two digits for
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  string, conventionally plus or minus followed by two digits for
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  hours and two for minutes: the standards say that an empty string
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  hours and two for minutes: the standards say that an empty string
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  should be output if the offset is unknown, but some systems use the
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  should be output if the offset is unknown, but some systems use the
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  \code{"\%a \%b \%e \%H:\%M:\%S \%Y"}.  As Windows does not comply (and
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  \code{"\%a \%b \%e \%H:\%M:\%S \%Y"}.  As Windows does not comply (and
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  uses a date format not understood outside N. America), that format is
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  uses a date format not understood outside N. America), that format is
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  used by \R on Windows in all locales.
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  used by \R on Windows in all locales.
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}
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}
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\section{Sources}{
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  Input uses the POSIX function \code{strptime} and output the C99
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  function \code{strftime}.
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  However, not all OSes (notably Windows) provided \code{strptime} and
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  many issues were found for those which did, so since 2000 \R has used
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  a fork of code from \samp{glibc}.  The forked code uses the
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  system's \code{strftime} to find the locale-specific day and month
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  names and any AM/PM indicator.
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  On some platforms (including Windows and by default on macOS) the
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  system's \code{strftime} is replaced (along with most of the rest of
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  the C-level datetime code) by code modified from IANA's \samp{tzcode}
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  distribution (\url{https://www.iana.org/time-zones}).
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}
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\references{
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\references{
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  International Organization for Standardization (2004, 2000, \dots)
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  International Organization for Standardization (2004, 2000, \dots)
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  \emph{ISO 8601.  Data elements and interchange formats --
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  \emph{ISO 8601.  Data elements and interchange formats --
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    Information interchange -- Representation of dates and times.}
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    Information interchange -- Representation of dates and times.}
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  For links to versions available on-line see (at the time of writing)
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  For links to versions available on-line see (at the time of writing)
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## time to sub-second accuracy (if supported by the OS)
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## time to sub-second accuracy (if supported by the OS)
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format(Sys.time(), "\%H:\%M:\%OS3")
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format(Sys.time(), "\%H:\%M:\%OS3")
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## read in date info in format 'ddmmmyyyy'
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## read in date info in format 'ddmmmyyyy'
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## This will give NA(s) in some locales; setting the C locale
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## This will give NA(s) in some non-English locales; setting the C locale
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## as in the commented lines will overcome this on most systems.
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## as in the commented lines will overcome this on most systems.
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## lct <- Sys.getlocale("LC_TIME"); Sys.setlocale("LC_TIME", "C")
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## lct <- Sys.getlocale("LC_TIME"); Sys.setlocale("LC_TIME", "C")
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x <- c("1jan1960", "2jan1960", "31mar1960", "30jul1960")
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x <- c("1jan1960", "2jan1960", "31mar1960", "30jul1960")
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z <- strptime(x, "\%d\%b\%Y")
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z <- strptime(x, "\%d\%b\%Y")
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## Sys.setlocale("LC_TIME", lct)
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## Sys.setlocale("LC_TIME", lct)
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z # prints without fractional seconds
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z # prints without fractional seconds
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op <- options(digits.secs = 3)
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op <- options(digits.secs = 3)
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z
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z
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options(op)
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options(op)
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## time zones name are not portable, but 'EST5EDT' comes pretty close.
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## time zone names are not portable, but 'EST5EDT' comes pretty close.
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(x <- strptime(c("2006-01-08 10:07:52", "2006-08-07 19:33:02"),
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(x <- strptime(c("2006-01-08 10:07:52", "2006-08-07 19:33:02"),
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               "\%Y-\%m-\%d \%H:\%M:\%S", tz = "EST5EDT"))
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               "\%Y-\%m-\%d \%H:\%M:\%S", tz = "EST5EDT"))
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attr(x, "tzone")
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attr(x, "tzone")
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## An RFC 5322 header (Eastern Canada, during DST)
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## An RFC 5322 header (Eastern Canada, during DST)