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Shifting: date and date-time

Source: R/posixt.R
posixt-shifting.Rd

date_shift() shifts x to the target weekday. You can shift to the next or previous weekday. If x is currently on the target weekday, you can choose to leave it alone or advance it to the next instance of the target.

Shifting with date-times retains the time of day where possible. Be aware that you can run into daylight saving time issues if you shift into a daylight saving time gap or fallback period.

Usage

# S3 method for class 'POSIXt'
date_shift(
  x,
  target,
  ...,
  which = "next",
  boundary = "keep",
  nonexistent = NULL,
  ambiguous = x
)

Arguments

x

[POSIXct / POSIXlt]

A date-time vector.

target

[weekday]

A weekday created from weekday() to target.

Generally this is length 1, but can also be the same length as x.

...

These dots are for future extensions and must be empty.

which

[character(1)]

One of:

  • "next": Shift to the next instance of the target weekday.

  • "previous: Shift to the previous instance of the target weekday.

boundary

[character(1)]

One of:

  • "keep": If x is currently on the target weekday, return it.

  • "advance": If x is currently on the target weekday, advance it anyways.

nonexistent

[character / NULL]

One of the following nonexistent time resolution strategies, allowed to be either length 1, or the same length as the input:

  • "roll-forward": The next valid instant in time.

  • "roll-backward": The previous valid instant in time.

  • "shift-forward": Shift the nonexistent time forward by the size of the daylight saving time gap.

  • "shift-backward: Shift the nonexistent time backward by the size of the daylight saving time gap.

  • "NA": Replace nonexistent times with NA.

  • "error": Error on nonexistent times.

Using either "roll-forward" or "roll-backward" is generally recommended over shifting, as these two strategies maintain the relative ordering between elements of the input.

If NULL, defaults to "error".

If getOption("clock.strict") is TRUE, nonexistent must be supplied and cannot be NULL. This is a convenient way to make production code robust to nonexistent times.

ambiguous

[character / zoned_time / POSIXct / list(2) / NULL]

One of the following ambiguous time resolution strategies, allowed to be either length 1, or the same length as the input:

  • "earliest": Of the two possible times, choose the earliest one.

  • "latest": Of the two possible times, choose the latest one.

  • "NA": Replace ambiguous times with NA.

  • "error": Error on ambiguous times.

Alternatively, ambiguous is allowed to be a zoned_time (or POSIXct) that is either length 1, or the same length as the input. If an ambiguous time is encountered, the zoned_time is consulted. If the zoned_time corresponds to a naive_time that is also ambiguous and uses the same daylight saving time transition point as the original ambiguous time, then the offset of the zoned_time is used to resolve the ambiguity. If the ambiguity cannot be resolved by consulting the zoned_time, then this method falls back to NULL.

Finally, ambiguous is allowed to be a list of size 2, where the first element of the list is a zoned_time (as described above), and the second element of the list is an ambiguous time resolution strategy to use when the ambiguous time cannot be resolved by consulting the zoned_time. Specifying a zoned_time on its own is identical to list(<zoned_time>, NULL).

If NULL, defaults to "error".

If getOption("clock.strict") is TRUE, ambiguous must be supplied and cannot be NULL. Additionally, ambiguous cannot be specified as a zoned_time on its own, as this implies NULL for ambiguous times that the zoned_time cannot resolve. Instead, it must be specified as a list alongside an ambiguous time resolution strategy as described above. This is a convenient way to make production code robust to ambiguous times.

Value

x shifted to the target weekday.

Examples

tuesday <- weekday(clock_weekdays$tuesday)

x <- as.POSIXct("1970-04-22 02:30:00", "America/New_York")

# Shift to the next Tuesday
date_shift(x, tuesday)
#> [1] "1970-04-28 02:30:00 EDT"

# Be aware that you can run into daylight saving time issues!
# Here we shift directly into a daylight saving time gap
# from 01:59:59 -> 03:00:00
sunday <- weekday(clock_weekdays$sunday)
try(date_shift(x, sunday))
#> Error in as_zoned_time(x, zone = tz, nonexistent = nonexistent, ambiguous = ambiguous) : 
#>   Nonexistent time due to daylight saving time at location 1.
#> ℹ Resolve nonexistent time issues by specifying the `nonexistent` argument.

# You can resolve this with the `nonexistent` argument
date_shift(x, sunday, nonexistent = "roll-forward")
#> [1] "1970-04-26 03:00:00 EDT"