| as.POSIX* {base} | R Documentation |
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 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.
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