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

Description

round_date() takes a date-time object and time unit, and rounds it to the nearest value of the specified time unit. For rounding date-times which are 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", as per IEC 60559.

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

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

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

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 multiples of units (except weeks) is supported.

week_start

when unit is weeks, specify the reference day. 7 represents Sunday and 1 represents Monday.

change_on_boundary

if this is NULL (the default), instants on the boundary remain unchanged, but Date objects are rounded up to the next boundary. If this is TRUE, instants on the boundary are rounded up to the next boundary. If this is FALSE, nothing on the boundary is rounded up at all. This was the default for lubridate prior to v1.6.0. See section Rounding Up Date Objects below for more details.

Rounding Up Date Objects

By default, rounding up Date objects follows 3 steps:

Convert to an instant representing lower bound of the Date: 2000-01-01 --> 2000-01-01 00:00:00
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 a day to its lower boundary.

Behavior on the boundary can be changed by setting change_on_boundary to TRUE or FALSE.
If the rounding unit is smaller than a day, return the instant from step 2 (POSIXct), otherwise convert to and return a Date object.

Details

In lubridate, functions that round date-time objects try 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 as per usual R conventions.

Examples

Run this code

# NOT RUN {
## 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"

## behaviour of `change_on_boundary`
## As per default behaviour `NULL`, instants on the boundary remain the
## same but dates are rounded up
ceiling_date(ymd_hms("2000-01-01 00:00:00"), "month")
ceiling_date(ymd("2000-01-01"), "month")

## If `TRUE`, both instants and dates on the boundary are rounded up
ceiling_date(ymd_hms("2000-01-01 00:00:00"), "month", change_on_boundary = TRUE)
ceiling_date(ymd("2000-01-01"), "month")

## If `FALSE`, both instants and dates on the boundary remain the same
ceiling_date(ymd_hms("2000-01-01 00:00:00"), "month", change_on_boundary = FALSE)
ceiling_date(ymd("2000-01-01"), "month")


 x <- ymd_hms("2000-01-01 00:00:00")
 ceiling_date(x, "month")
 ceiling_date(x, "month", change_on_boundary = TRUE)

 ## For Date objects first day of the month is not on the
 ## "boundary". change_on_boundary applies to instants only.
 x <- ymd("2000-01-01")
 ceiling_date(x, "month")
 ceiling_date(x, "month", change_on_boundary = TRUE)
# }