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"', '"date"' (from package 'date' or 'survival'), '"chron"'
     and '"dates"' (from package 'chron') to these classes.  Dates are
     treated as being at midnight UTC.

     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'.)

     Logical 'NA's can be converted to either of the classes, but no
     other logical vectors can be.

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

