% File src/library/base/man/Dates.Rd
% Part of the R package, https://www.R-project.org
% Copyright 1995-2024 R Core Team
% Distributed under GPL 2 or later

\name{Dates}
\alias{Date}
\alias{Dates}
\alias{print.Date}
\alias{summary.Date}
\alias{Math.Date}
\alias{Summary.Date}
\alias{[.Date}
\alias{[<-.Date}
\alias{length<-.Date}
\alias{[[.Date}
\alias{as.list.Date}
\alias{c.Date}
\alias{mean.Date}
\alias{split.Date}

\title{Date Class}
\description{
  Description of the class \code{"Date"} representing calendar dates.
}
\usage{
\method{summary}{Date}(object, digits = 12, \dots)

\method{print}{Date}(x, max = NULL, \dots)
% not sure if we should not rather _drop_ the 'recursive'!
%\method{c}{Date}(\dots, recursive = FALSE)
}
\arguments{
  \item{object, x}{a \code{Date} object to be summarized or printed.}
  \item{digits}{number of significant digits for the computations.}
  \item{max}{numeric or \code{NULL}, specifying the maximal number of
    entries to be printed.  By default, when \code{NULL},
    \code{\link{getOption}("max.print")} used.}
  \item{\dots}{further arguments to be passed from or to other methods.}
  %% \item{\dots, recursive}{further arguments to be passed from or to
  %%   other methods; \code{recursive} is not considered in \code{Date} method.}
}
\details{
  Dates are represented as the number of days since 1970-01-01, with
  negative values for earlier dates.  They are always printed
  following the rules of the current Gregorian calendar, even though
  that calendar was not in use long ago (it was adopted in 1752 in
  Great Britain and its colonies).  When printing there is assumed to
  be a year zero.
  % man cal on Linux refers to this as 'the British empire'
  
  It is intended that the date should be an integer value, but this is
  not enforced in the internal representation.  Fractional days will be
  ignored when printing.  It is possible to produce fractional days via
  the \code{mean} method or by adding or subtracting (see
  \code{\link{Ops.Date}}).

  When a date is converted to a date-time (for example by
  \code{\link{as.POSIXct}} or \code{\link{as.POSIXlt}} its time is taken
  as midnight in UTC.

  Printing dates involves conversion to class \code{"\link{POSIXlt}"}
  which treats dates of more than about 780 billion days (2.1 billion
  years) from present as \code{NA}.
  
  For the many methods see \code{methods(class = "Date")}.  Several are
  documented separately, see below.
}
\section{Warning}{
  Do not use \code{\link{identical}()} on objects of class \code{"Date"}.
  Their \code{\link{storage.mode}} may be \code{"double"} or
  \code{"integer"}, and which is chosen has depended on the version of
  \R used to create the object.
}
\seealso{
  \code{\link{Sys.Date}} for the current date.

  \code{\link{weekdays}} for convenience extraction functions.

  Methods with extra arguments and documentation:
  \itemize{
    \item \code{\link{Ops.Date}} for operators on \code{"Date"} objects.
    \item \code{\link{format.Date}} for conversion to and from character
    strings.
    \item \code{\link{axis.Date}} and \code{\link{hist.Date}} for plotting.
    \item \code{\link{seq.Date}}, \code{\link{cut.Date}} and
    \code{\link{round.Date}} for utility operations.
  }
  \code{\link{DateTimeClasses}} for date-time classes.
}
\examples{\donttest{
(today <- Sys.Date())
format(today, "\%d \%b \%Y")  # with month as a word
(tenweeks <- seq(today, length.out=10, by="1 week")) # next ten weeks
weekdays(today)
months(tenweeks)
}% dont
(Dls <- as.Date(.leap.seconds))

## Show use of year zero:
(z <- as.Date("01-01-01")) # how it is printed depends on the OS
z - 365 # so year zero was a leap year.
as.Date("00-02-29")
# if you want a different format, consider something like (if supported)
\dontrun{format(z, "\%04Y-\%m-\%d") # "0001-01-01"
format(z, "\%_4Y-\%m-\%d") # "   1-01-01"
format(z, "\%_Y-\%m-\%d")  # "1-01-01"
} % these are not supported by macOS's native strftimw.

##  length(<Date>) <- n   now works
ls <- Dls; length(ls) <- 12
l2 <- Dls; length(l2) <- 5 + length(Dls)
stopifnot(exprs = {
  ## length(.) <- * is compatible to subsetting/indexing:
  identical(ls, Dls[seq_along(ls)])
  identical(l2, Dls[seq_along(l2)])
  ## has filled with NAs
  is.na(l2[(length(Dls)+1):length(l2)])
})
}
\keyword{utilities}
\keyword{chron}
