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.

### Usage

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

    ## S3 method for class 'character':
    as.POSIXlt(x, tz = "", format, ...)

    ## S3 method for class 'numeric':
    as.POSIXlt(x, tz = "", origin, ...)

    ## 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 (see time zones), but `""` is the current timezone, and `"GMT"` is UTC (Universal Time, Coordinated).
`...` | further arguments to be passed to or from other methods.
`format` | character string giving a date-time format as used by `strptime`.
`origin` | a date-time object, or something which can be coerced by `as.POSIXct(tz="GMT")` to such an object.

### 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. Alternatively, `format` can be specified for character vectors or factors.

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.

### 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. `Sys.timezone` for details of the (system-specific)naming of time zones.

### 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
    # ways to convert this
    as.POSIXct(z, origin="1960-01-01")                # local
    as.POSIXct(z, origin="1960-01-01", tz="GMT")      # in UTC
    as.POSIXct(z, origin=ISOdatetime(1960,1,1,0,0,0)) # local
    ISOdatetime(1960,1,1,0,0,0) + z                   # local

    ## SPSS dates (R-help 2006-02-17)
    z <- c(10485849600, 10477641600, 10561104000, 10562745600)
    as.Date(as.POSIXct(z, origin="1582-10-14", tz="GMT"))

    as.POSIXlt(Sys.time(), "GMT") # the current time in UTC
    ## Not run:
    ## These may not be correct names on your system
    as.POSIXlt(Sys.time(), "America/New_York")  # in New York
    as.POSIXlt(Sys.time(), "EST5EDT")           # alternative.
    as.POSIXlt(Sys.time(), "EST" )   # somewhere in Eastern Canada
    as.POSIXlt(Sys.time(), "HST")    # in Hawaii
    as.POSIXlt(Sys.time(), "Australia/Darwin")
    ## End(Not run)

[Package base version 2.8.1 Index]