\Header{DateTimeClasses}{Date-Time Classes}
\alias{+.POSIXct}{DateTimeClasses}
\alias{+.POSIXlt}{DateTimeClasses}
\alias{-.POSIXct}{DateTimeClasses}
\alias{-.POSIXlt}{DateTimeClasses}
\alias{.leap.seconds}{DateTimeClasses}
\alias{all.equal.POSIXct}{DateTimeClasses}
\alias{as.data.frame.POSIXct}{DateTimeClasses}
\alias{as.matrix.POSIXlt}{DateTimeClasses}
\alias{c.POSIXct}{DateTimeClasses}
\alias{c.POSIXlt}{DateTimeClasses}
\alias{is.na.POSIXlt}{DateTimeClasses}
\alias{Math.POSIXct}{DateTimeClasses}
\alias{Math.POSIXlt}{DateTimeClasses}
\alias{mean.POSIXct}{DateTimeClasses}
\alias{mean.POSIXlt}{DateTimeClasses}
\alias{Ops.POSIXct}{DateTimeClasses}
\alias{Ops.POSIXlt}{DateTimeClasses}
\alias{POSIXct}{DateTimeClasses}
\alias{POSIXlt}{DateTimeClasses}
\alias{print.POSIXct}{DateTimeClasses}
\alias{print.POSIXlt}{DateTimeClasses}
\alias{Summary.POSIXct}{DateTimeClasses}
\alias{summary.POSIXct}{DateTimeClasses}
\alias{Summary.POSIXlt}{DateTimeClasses}
\alias{summary.POSIXlt}{DateTimeClasses}
\alias{[.POSIXct}{DateTimeClasses}
\alias{[<\Rdash.POSIXct}{DateTimeClasses}
\alias{[[.POSIXct}{DateTimeClasses}
\keyword{utilities}{DateTimeClasses}
\keyword{chron}{DateTimeClasses}
\begin{Description}\relax
Description of the classes \code{"POSIXlt"} and \code{"POSIXct"}
representing calendar dates and times (to the nearest second).\end{Description}
\begin{Usage}
\begin{verbatim}
print.POSIXct(x, ...)
print.POSIXlt(x, ...)

summary.POSIXct(object, digits = 15, ...)
summary.POSIXlt(object, digits = 15, ...)

time + number
time - number
time1 lop time2

c(time1, ...)
mean(time)
\end{verbatim}
\end{Usage}
\begin{Arguments}
\begin{ldescription}
\item[\code{x, object}] An object to be printed or summarized.
\item[\code{digits}] Number of significant digits for the computations:
should be high enough to represent the least important time unit
exactly.
\item[\code{...}] Further arguments to be passed from or to other methods.
\item[\code{time, time1, time2, ...}] date-time objects.
\item[\code{number}] a numeric object.
\item[\code{lop}] One of \code{==}, \code{!=}, \code{<}, \code{<=}, \code{>}
or \code{>=}.
\end{ldescription}
\end{Arguments}
\begin{Details}\relax
There are two basic classes of date/times.  Class \code{"POSIXct"}
represents the (signed) number of seconds since the beginning of 1970
as a numeric vector.  Class \code{"POSIXlt"} is a named list of
vectors representing
\describe{
\item[\code{sec}] 0--61: seconds
\item[\code{min}] 0--59: minutes
\item[\code{hour}] 0--23: hours
\item[\code{mday}] 1--31: day of the month
\item[\code{mon}] 0--11: months after the first of the year.
\item[\code{year}] Years since 1900.
\item[\code{wday}] 0--6 day of the week, starting on Sunday.
\item[\code{yday}] 0--365: day of the year.
\item[\code{isdst}] Daylight savings time flag. Positive if in
force, zero if not, negative if unknown.
}
The classes correspond to the ANSI C constructs of ``calendar time''
(the \code{time\_t} data type) and ``local time'' (or broken-down
time, the \code{struct tm} data type), from which they also inherit
their names.

\code{"POSIXct"} is more convenient for including in data frames, and
\code{"POSIXlt"} is closer to human-readable forms.  Logical
comparisons and limited arithmetic are available for both classes.
Differences between objects are returned as the number of (non-leap)
seconds between them.  One can add or subtract a number of seconds
from a date-time object, but not add two date-time objects.

\code{"POSIXlt"} objects will often have an attribute \code{"tzone"},
a character vector of length 3 giving the timezone name from the
\code{"TZ"} environment variable and the names of the base timezone
and the alternate (daylight-saving) timezone.  Sometimes this may
just be of length one, giving the timezone name.

Unfortunately, the conversion is complicated by the operation of time
zones and leap seconds (22 days have been 86401 seconds long so far:
the times of the extra seconds are in the object \code{.leap.seconds}).
The details of this are entrusted to the OS services where possible.
This will usually cover the period 1970--2037, and on Unix machines
back to 1902 (when time zones were in their infancy).  Outside those
ranges we use our own C code.  This uses the offset from GMT in use
in the timezone in 2000, and uses the alternate (daylight-saving)
timezone only if \code{isdst} is positive.

It seems that some systems use leap seconds but most do not. This is
detected and corrected for at build time, so all \code{"POSIXct"}
times used by \R{} do not include leap seconds.  (Conceivably this could
be wrong if the system has changed since build time, just possibly by
changing locales.)

Using \code{c} on \code{"POSIXlt"} objects converts them to the
current time zone.\end{Details}
\begin{Section}{Warning}
Some Unix-like systems (especially Linux ones) do not have \code{"TZ"}
set, yet have internal code that expects it (as does POSIX).  We have
tried to work around this, but if you get unexpected results try
setting \code{"TZ"}.\end{Section}
\begin{SeeAlso}\relax
\code{\Link{as.POSIXct}} and \code{\Link{as.POSIXlt}} for conversion
between the classes.

\code{\Link{strptime}} for conversion to and from character
representations.

\code{\Link{Sys.time}} for clock time as a \code{"POSIXct"} object.\end{SeeAlso}
\begin{Examples}
\begin{ExampleCode}
(z <- Sys.time())             # the current date, as class "POSIXct"

(z <- Sys.time() - 3600)      # an hour ago
Sys.time() - z                # just over 3600 seconds.

as.POSIXlt(Sys.time(), "GMT") # the current time in GMT
format(.leap.seconds)         # all 22 leapseconds in your timezone
\end{ExampleCode}
\end{Examples}

