R: Date-Time Classes

DateTimeClasses {base} | R Documentation

## Date-Time Classes

### Description

Description of the classes `"POSIXlt"` and `"POSIXct"` representing calendar dates and times (to the nearest second).

### Usage

    print.POSIXct(x, ...)
    print.POSIXlt(x, ...)

    summary.POSIXct(object, digits = 15, ...)
    summary.POSIXlt(object, digits = 15, ...)

    time + number
    time - number
    time1 lop time2

### Arguments

`x, object` | An object to be printed or summarized.
`digits` | Number of significant digits for the computations: should be high enough to represent the least important time unit exactly.
`...` | Further arguments to be passed from or to other methods.
`time, time1, time2` | date-time objects.
`number` | a numeric object.
`lop` | One of `==`, `!=`, `<`, `<=`, `>` or `>=`.

### Details

There are two basic classes of date/times. Class `"POSIXct"` represents the (signed) number of seconds since the beginning of 1970 as a numeric vector. Class `"POSIXlt"` is a named list of vectors representing

`sec`
  0–61: seconds
`min`
  0–59: minutes
`hour`
  0–23: hours
`mday`
  1–31: day of the month
`mon`
  0–11: months after the first of the year.
`year`
  Years since 1900.
`wday`
  0–6 day of the week, starting on Sunday.
`yday`
  0–365: day of the year.
`isdst`
  Daylight savings time flag. Positive if in force, zero if not, negative if unknown.

The classes correspond to the ANSI C constructs of ``calendar time'' (the `time\_t` data type) and ``local time'' (or broken-down time, the `struct tm` data type), from which they also inherit their names.

`"POSIXct"` is more convenient for including in data frames, and `"POSIXlt"` is closer to human-readable forms. A virtual class `"POSIXt"` inherits from both of the classes: it is used to allow operations such as subtraction to mix the two classes.

Logical comparisons and limited arithmetic are available for both classes. One can add or subtract a number of seconds or a `difftime` object from a date-time object, but not add two date-time objects. Subtraction of two date-time objects is equivalent to using `difftime`. Be aware that `"POSIXlt"` objects will be interpreted as being in the current timezone for these operations, unless a timezone has been specified.

`"POSIXlt"` objects will often have an attribute `"tzone"`, a character vector of length 3 giving the timezone name from the `"TZ"` environment variable and the names of the base timezone and the alternate (daylight-saving) timezone. Sometimes this may just be of length one, giving the timezone name.

Unfortunately, the conversion is complicated by the operation of time zones and leap seconds (22 days have been 86401 seconds long so far: the times of the extra seconds are in the object `.leap.seconds`). The details of this are entrusted to the OS services where possible. This will usually cover the period 1970–2037, and on Unix machines back to 1902 (when time zones were in their infancy). Outside those ranges we use our own C code. This uses the offset from GMT in use in the timezone in 2000, and uses the alternate (daylight-saving) timezone only if `isdst` is positive.

It seems that some systems use leap seconds but most do not. This is detected and corrected for at build time, so all `"POSIXct"` times used by R do not include leap seconds. (Conceivably this could be wrong if the system has changed since build time, just possibly by changing locales.)

Using `c` on `"POSIXlt"` objects converts them to the current time zone.

### Warning

Some Unix-like systems (especially Linux ones) do not have `"TZ"` set, yet have internal code that expects it (as does POSIX). We have tried to work around this, but if you get unexpected results try setting `"TZ"`.

### See Also

`as.POSIXct` and `as.POSIXlt` for conversion between the classes.

`strptime` for conversion to and from character representations.

`Sys.time` for clock time as a `"POSIXct"` object.

`difftime` for time intervals.

`cut.POSIXt`, `seq.POSIXt`, `round.POSIXt` and `trunc.POSIXt` for methods for these classes.

`weekdays.POSIXt` for convenience extraction functions.

### Examples

    (z <- Sys.time())             # the current date, as class "POSIXct"

    Sys.time() - 3600             # an hour ago

    as.POSIXlt(Sys.time(), "GMT") # the current time in GMT
    format(.leap.seconds)         # all 22 leapseconds in your timezone

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