as.POSIX* {base}R Documentation

Date-time Conversion Functions


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


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


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).


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 NAs can be converted to either of the classes, but no other logical vectors can be.


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


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.


(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

[Package base version 2.1.0 Index]