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.

Usage:

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

     ## S3 method for class 'POSIXlt':
     as.double(x, ...)

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).

     ...: further arguments to be passed to or from other methods.

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 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 without
     times 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'.)  Fractional seconds are
     allowed.

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

     The 'as.double' method converts '"POSIXlt"' objects to
     '"POSIXct"'.

     If you are given a numeric time as the number of seconds since an
     epoch, see the examples.

     Where OSes describe their valid timezones can be obscure.  The
     help for 'tzset' (or '_tzset' on Windows) can be helpful, but it
     can also be inaccurate.  There is a cumbersome POSIX specification
     (listed under environment variable 'TZ' at <URL:
     http://www.opengroup.org/onlinepubs/009695399/basedefs/xbd_chap08.html>),
     which is often at least partially supported, but there may be
     other more user-friendly ways to specify timezones. For most
     Unix-alikes (including MacOS X) this can be an optional colon
     prepended to the path to a file (by default under
     '/usr/share/zoneinfo' or '/usr/lib/zoneinfo' (or even
     '/usr/share/lib/zoneinfo' on Solaris)), for example 'EST5EDT' or
     'GB' or 'Europe/Paris'.  See <URL:
     http://www.twinsun.com/tz/tz-link.htm> for more details and
     references.

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 datetime, 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 datetime, as class "POSIXlt"
     unlist(unclass(z))            # a list shown as a named vector

     ## suppose we have a time in seconds since 1960-01-01 00:00:00 GMT
     z <- 1472562988
     # two ways to convert this
     ISOdatetime(1960,1,1,0,0,0) + z # late August 2006
     strptime("1960-01-01", "%Y-%m-%d", tz="GMT") + z

     as.POSIXlt(Sys.time(), "GMT") # the current time in GMT
     ## Not run: 
     ## These may not be correct names on your system
     as.POSIXlt(Sys.time(), "EST5EDT")  # the current time in New York
     as.POSIXlt(Sys.time(), "EST" )     # ditto, ignoring DST
     as.POSIXlt(Sys.time(), "HST")      # the current time in Hawaii
     as.POSIXlt(Sys.time(), "Australia/Darwin")
     ## End(Not run)

