R: Date-time Conversion Functions

as.POSIX* {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

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