as.POSIX*                package:base                R Documentation

Date-time Conversion Functions

Description:

     Functions to manipulate objects of classes `"POSIXlt"' and
     `"POSIXct"' representing calendar dates and times (to the nearest
     second).

Usage:

     as.POSIXct(x, tz = "")
     as.POSIXlt(x, tz = "")

Arguments:

       x: An object to be converted.

      tz: A timezone specification to be used for the conversion, if
          one is required.  System-specific, but `""' is the current
          timezone, and `"GMT"' is UTC (Coordinated Universal Time, in
          French).

Details:

     The `as.POSIX*' functions convert an object to one of the two
     classes used to represent date/times (calendar dates plus time to
     the nearest second).  They can take convert a wide variety of
     objects, including objects of the other class and of classes
     `"date"' (from package `date'), `"chron"' and `"dates"' (from
     package `chron') to these classes.  They can also convert
     character strings of the formats `"2001-02-03"' and `"2001/02/03"'
     optionally followed by white space and a time in the format
     `"14:52"' or `"14:52:03"'.  (Formats such as `"01/02/03"' are
     ambiguous but can be converted via a format specification by
     `strptime'.)

Value:

     `as.POSIXct' and `as.POSIXlt' return an object of the appropriate
     class.  If `tz' was specified, `as.POSIXlt' will give an
     appropriate `"tzone"' attribute.

Note:

     If you want to extract specific aspects of a time (such as the day
     of the week) just convert it to class `"POSIXlt"' and extract the
     relevant component(s) of the list, or if you want a character
     representation (such as a named day of the week) use
     `format.POSIXlt' or `format.POSIXct'.

     If a timezone is needed and that specified is invalid on your
     system, what happens is system-specific but it will probably be
     ignored.

See Also:

     DateTimeClasses for details of the classes; `strptime' for
     conversion to and from character representations.

Examples:

     (z <- Sys.time())             # the current date, as class "POSIXct"
     unclass(z)                    # a large integer
     floor(unclass(z)/86400)       # the number of days since 1970-01-01

     (z <- as.POSIXlt(Sys.time())) # the current date, as class "POSIXlt"
     unlist(unclass(z))            # a list shown as a named vector

     as.POSIXlt(Sys.time(), "GMT") # the current time in GMT

