time_add: Arithmetics with periods

Description

Add periods to date-time objects. Periods track the change in the "clock time" between two civil times. They are measured in common civil time units: years, months, days, hours, minutes, and seconds.

Arithmetic operations with multiple period units (years, months etc) are applied in decreasing size order, from year to second. Thus time_add(x, months = 1, days = 3) first adds 1 to x and then 3 days.

Generally period arithmetic is undefined due to the irregular nature of civil time and complexities with DST transitions. `timechange` allows for a refined control of what happens when an addition of irregular periods (years, months, days) results in "unclear" date.

Let's start with an example. What happens when you add "1 month 3 days" to "2000-01-31 01:02:03"? `timechange` operates by applying larger periods first. First months are added1 + 1 = February which results in non-existent time of 2000-02-31 01:02:03. Here the roll_month adjustment kicks in:

skip - no adjustment is done to the simple arithmetic operations (the gap is skipped as if it's not there. Thus, 2000-01-31 01:02:03 + 1 month + 3 days is equivalent to 2000-01-01 01:02:03 + 1 month + 31 days + 3 days resulting in 2000-03-05 01:02:03.
NA - if any of the intermediate additions result in non-existent dates NA is produced. This is how arithmetic in lubridate operates.
boundary - if an intermediate computation falls in a gap, the date is adjusted to the next valid time. Thus, 2000-01-31 01:02:03 + month = 2000-03-01 00:00:00.
next - is like boundary but preserves the smaller units. Thus, 2000-01-31 01:02:03 + 1 month = 2000-03-01 01:02:03.
prev - is like next but instead of rolling forward to the first day of the month, it rolls back to the last valid day of the previous month. Thus, 2000-01-31 01:02:03 + 1 month = 2000-02-28 01:02:03. This is the default.

Usage

time_add(
  time,
  periods = NULL,
  years = NULL,
  months = NULL,
  weeks = NULL,
  days = NULL,
  hours = NULL,
  minutes = NULL,
  seconds = NULL,
  roll_month = "last",
  roll_dst = "first"
)
time_subtract(
  time,
  periods = NULL,
  years = NULL,
  months = NULL,
  weeks = NULL,
  days = NULL,
  hours = NULL,
  minutes = NULL,
  seconds = NULL,
  roll_month = "last",
  roll_dst = "last"
)

Arguments

time: date-time object
periods: string of units to add/subtract (not yet implemented) or a named list of the form list(years = 1, months = 2, ...).
years, months, weeks, days, hours, minutes, seconds: Units to be added to time. Each unit except for seconds must be expressed as integer values.
roll_month: controls how addition of months and years behaves when standard arithmetic rules exceed limits of the resulting date's month. See "Details" for the description of possible values.
roll_dst: controls how to adjust the updated time if it falls within a DST transition intervals. See the "Details".

Examples

Run this code


# Addition

## Month gap
x <- as.POSIXct("2000-01-31 01:02:03", tz = "America/Chicago")
time_add(x, months = 1, roll_month = "first")
time_add(x, months = 1, roll_month = "last")
time_add(x, months = 1, roll_month = "boundary")
time_add(x, months = 1, roll_month = "skip")
time_add(x, months = 1, roll_month = "NA")
time_add(x, months = 1, days = 3,  roll_month = "first")
time_add(x, months = 1, days = 3,  roll_month = "last")
time_add(x, months = 1, days = 3,  roll_month = "boundary")
time_add(x, months = 1, days = 3,  roll_month = "skip")
time_add(x, months = 1, days = 3,  roll_month = "NA")

## DST gap
x <- as.POSIXlt("2010-03-14 01:02:03", tz = "America/Chicago")
time_add(x, hours = 1, minutes = 50, roll_dst = "first")
time_add(x, hours = 1, minutes = 50, roll_dst = "last")
time_add(x, hours = 1, minutes = 50, roll_dst = "boundary")
time_add(x, hours = 1, minutes = 50, roll_dst = "skip")
time_add(x, hours = 1, minutes = 50, roll_dst = "NA")

# SUBTRACTION

## Month gap
x <- as.POSIXct("2000-03-31 01:02:03", tz = "America/Chicago")
time_subtract(x, months = 1, roll_month = "first")
time_subtract(x, months = 1, roll_month = "last")
time_subtract(x, months = 1, roll_month = "boundary")
time_subtract(x, months = 1, roll_month = "skip")
time_subtract(x, months = 1, roll_month = "NA")
time_subtract(x, months = 1, days = 3,  roll_month = "first")
time_subtract(x, months = 1, days = 3,  roll_month = "last")
time_subtract(x, months = 1, days = 3,  roll_month = "boundary")
time_subtract(x, months = 1, days = 3,  roll_month = "skip")
time_subtract(x, months = 1, days = 3,  roll_month = "NA")

## DST gap
y <- as.POSIXlt("2010-03-15 01:02:03", tz = "America/Chicago")
time_subtract(y, hours = 22, minutes = 50, roll_dst = "first")
time_subtract(y, hours = 22, minutes = 50, roll_dst = "last")
time_subtract(y, hours = 22, minutes = 50, roll_dst = "boundary")
time_subtract(y, hours = 22, minutes = 50, roll_dst = "skip")
time_subtract(y, hours = 22, minutes = 50, roll_dst = "NA")

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