R: Round, floor and ceiling methods for date-time objects

round_date {lubridate} | R Documentation

## Round, floor and ceiling methods for date-time objects

### Description

Rounding to the nearest unit or multiple of a unit are supported. All meaningfull specifications in English language are supported - secs, min, mins, 2 minutes, 3 years etc.

Rounding to fractional seconds is supported. Please note that rounding to fractions smaller than 1s can lead to large precision errors due to the floating point representation of the POSIXct objects. See examples.

`round_date()` takes a date-time object and rounds it to the nearest value of the specified time unit. For rounding date-times which is exactly halfway between two consecutive units, the convention is to round up. Note that this is in line with the behavior of R's `base::round.POSIXt()` function but does not follow the convention of the base `base::round()` function which "rounds to the even digit" per IEC 60559.

`floor_date()` takes a date-time object and rounds it down to the nearest boundary of the specified time unit.

`ceiling_date()` takes a date-time object and rounds it up to the nearest boundary of the specified time unit.

### Usage

    round_date(x, unit = "second",
      week_start = getOption("lubridate.week.start", 7))

    floor_date(x, unit = "seconds",
      week_start = getOption("lubridate.week.start", 7))

    ceiling_date(x, unit = "seconds", change_on_boundary = NULL,
      week_start = getOption("lubridate.week.start", 7))

### Arguments

`x` | a vector of date-time objects
`unit` | a character string specifying a time unit or a multiple of a unit to be rounded to. Valid base units are `second`, `minute`, `hour`, `day`, `week`, `month`, `bimonth`, `quarter`, `season`, `halfyear` and `year`. Arbitrary unique English abbreviations as in the `period()` constructor are allowed. Rounding to multiple of units (except weeks) is supported.
`week_start` | when unit is `weeks` specify the reference day; 7 being Sunday.
`change_on_boundary` | If NULL (the default) don't change instants on the boundary (`ceiling_date(ymd_hms('2000-01-01 00:00:00'))` is `2000-01-01 00:00:00`), but round up `Date` objects to the next boundary (`ceiling_date(ymd("2000-01-01"), "month")` is `"2000-02-01"`). When `TRUE`, instants on the boundary are rounded up to the next boundary. When `FALSE`, date-time on the boundary are never rounded up (this was the default for lubridate prior to `v1.6.0`. See section `Rounding Up Date Objects` below for more details.

### Details

In lubridate, rounding of a date-time objects tries to preserve the class of the input object whenever possible. This is done by first rounding to an instant and then converting to the original class by usual R conventions.

### Rounding Up Date Objects

By default rounding up `Date` objects follows 3 steps:

1. Convert to an instant representing lower bound of the Date: `2000-01-01` –> `2000-01-01 00:00:00`

2. Round up to the next closest rounding unit boundary. For example, if the rounding unit is `month` then next closest boundary of `2000-01-01` is `2000-02-01 00:00:00`.

   The motivation for this is that the "partial" `2000-01-01` is conceptually an interval (`2000-01-01 00:00:00` – `2000-01-02 00:00:00`) and the day hasn't started clocking yet at the exact boundary `00:00:00`. Thus, it seems wrong to round up a day to its lower boundary.

   The behavior on the boundary can be changed by setting `change_on_boundary` to a non-`NULL` value.

3. If rounding unit is smaller than a day, return the instant from step 2 (`POSIXct`), otherwise convert to and return a `Date` object.

### See Also

`base::round()`

### Examples

    ## print fractional seconds
    options(digits.secs=6)

    x <- ymd_hms("2009-08-03 12:01:59.23")
    round_date(x, ".5s")
    round_date(x, "sec")
    round_date(x, "second")
    round_date(x, "minute")
    round_date(x, "5 mins")
    round_date(x, "hour")
    round_date(x, "2 hours")
    round_date(x, "day")
    round_date(x, "week")
    round_date(x, "month")
    round_date(x, "bimonth")
    round_date(x, "quarter") == round_date(x, "3 months")
    round_date(x, "halfyear")
    round_date(x, "year")

    x <- ymd_hms("2009-08-03 12:01:59.23")
    floor_date(x, ".1s")
    floor_date(x, "second")
    floor_date(x, "minute")
    floor_date(x, "hour")
    floor_date(x, "day")
    floor_date(x, "week")
    floor_date(x, "month")
    floor_date(x, "bimonth")
    floor_date(x, "quarter")
    floor_date(x, "season")
    floor_date(x, "halfyear")
    floor_date(x, "year")

    x <- ymd_hms("2009-08-03 12:01:59.23")
    ceiling_date(x, ".1 sec") # imprecise representation at 0.1 sec !!!
    ceiling_date(x, "second")
    ceiling_date(x, "minute")
    ceiling_date(x, "5 mins")
    ceiling_date(x, "hour")
    ceiling_date(x, "day")
    ceiling_date(x, "week")
    ceiling_date(x, "month")
    ceiling_date(x, "bimonth") == ceiling_date(x, "2 months")
    ceiling_date(x, "quarter")
    ceiling_date(x, "season")
    ceiling_date(x, "halfyear")
    ceiling_date(x, "year")

    ## As of R 3.4.2 POSIXct printing of fractional numbers is wrong
    as.POSIXct("2009-08-03 12:01:59.3") ## -> "2009-08-03 12:01:59.2 CEST"
    ceiling_date(x, ".1 sec") ## -> "2009-08-03 12:01:59.2 CEST"

    x <- ymd("2000-01-01")
    ceiling_date(x, "month")
    ceiling_date(x, "month", change_on_boundary = TRUE)

[Package lubridate version 1.7.2 Index]