aweek means 'any week


The day in which a week starts differs depending on context. For countries like the UK, the first day of the week is the first working day, which is Monday. This definition conforms with the ISO 8601 standard definition for the beginning of a week, but there are examples of situations where the first day of the week is different:

This package provides tools to convert dates to weeks and back where a week can start on any day. You can use this package for any of the following:

Converting dates to weeks

You can convert dates to weeks starting on any day by using date2week() with the week_start argument. This argument can be a number from 1 to 7 representing the ISO 8601 day of the week OR it can be a string representing the day of the week in either an English locale or the locale defined on your computer. The default of this argument is the value of get_week_start(), which is a thin wrapper around options("aweek.week_start", 1L). Unless you have specified a default aweek.week_start option with set_week_start(), this will always be set to 1 (Monday).

It is highly recommended that you set the default aweek.week_start either in the beginning of your Rscript, Rmarkdown document, or in your .Rprofile.


set_week_start("Sunday") # setting the default week_start to Sunday

dat <- as.Date("2019-03-03") + sample(-6:7, 10, replace = TRUE)
print(w <- date2week(dat))

If you need a different day on the fly, you can supply an integer or character day to the week_start argument.

# Use character days
date2week(dat, week_start = "Monday")
# Use ISO 8601 days
date2week(dat, week_start = 1)

If you want to save two extra keystrokes, you can also use the as.aweek() method for dates, which wraps date2week():

as.aweek(dat, week_start = 1)

What you get back is an aweek class object. It can be converted back to a date with either as.Date() or week2date():


How does it work?

The calculation of weeks from dates requires knowledge of the current day of the week and the number of days past 1 January.

Week numbers are calculated in three steps:

  1. Find the day of the week, relative to the week_start (d). The day of the week (d) relative to the week start (s) is calculated using the ISO week day (i) via d = 1L + ((i + (7L - s)) %% 7L).
  2. Find the date that represents midweek (m). The date that represents midweek is found by subtracting the day of the week (d) from 4 and adding that number of days to the current date: m = date + (4 - d).
  3. Find the week number (w) by counting the number of days since 1 January to (m), and use integer division by 7: w = 1L + ((m - yyyy-01-01) %/% 7)

For example, here's how to calculate the week for Tuesday, 6 December 2016, assuming the week start is a Sunday:

the_date <- as.Date("2016-12-06")
jan_1    <- as.Date("2016-01-01")

i <- as.POSIXlt(the_date)$wday # 2, the ISO date for Tuesday 
s <- 7L                        # week_start for sunday

# 1. Find the day of the week
print(d <- 1L + ((i + (7L - s)) %% 7L))

# 2. Find the date that represents midweek
print(m <- the_date + (4L - d))

# 3. Find the week
print(w <- 1L + as.integer(m - jan_1) %/% 7L)

# Format the week
sprintf("2016-W%02d-%d", w, d)

For the weeks around 1 January, the year is determined by the week number. If the month is January, but the week number is 52 or 53, then the year for the week (YYYY) is the calendar year (yyyy) minus 1. However, if the month is December, but the week number is 1, then the year for the week (YYYY) is the calendar year (yyyy) plus 1.

The aweek class

The result you see above is an object of class "aweek". The aweek class is a character that contains the week_start attribute. This attribute allows it to be easily converted back to a date without the user needing to enter the start day every time. You can convert a character that matches the YYYY-Www-d pattern to an aweek class object with as.aweek():

x <- as.aweek("2019-W10-1")

Under the hood, it checks the validity of the week string and then add the attribute and class:

x <- "2019-W10-1"
attr(x, "week_start") <- 7 # Sunday 
class(x) <- "aweek"

If you need to remove the class, you can just use as.character():


Best practices

The date2week() function only checks that dates are in ISO 8601 (yyyy-mm-dd) format before converting to weeks, and otherwise assumes that the dates are accurate so it's strongly recommended to make sure your dates are in either Date or POISXt format and accurate before converting to weeks. The lubridate can be used for this purpose.

Use set_week_start() at the beginning of all your scripts to explicitly define the day on which your weeks start. This can be overridden if need be in specific parts of your scripts. Otherwise, the default will be dependent on the value of getOption("aweek.week_start", 1L).

Because the week_start arguments default to get_week_start(), it's recommended to specify week_start in date2week() and week2date() if you don't have an aweek object.

Before you combine aweek objects, confirm that they are actually aweek objects with inherits(myObject, "aweek").

Weekly aggregation

There are times where you would want to aggregate your days into weeks, you can do this by specifying floor_day = TRUE in date2week(). For example, here we can show the individual weeks:

print(wf <- date2week(dat, week_start = "Saturday", floor_day = TRUE))

If you convert this to date, then all the dates will represent the beginning of the week:

print(dwf <- week2date(wf))

If you want to aggregate your aweek objects after you created them, you can always use the trunc() function:

w <- date2week(dat)


Weeks can be represented as factors, which is useful for tabulations across weeks. You can use factor = TRUE in date2week() and it will automatically fill in any missing weeks.

dat[1] + c(0, 15)
date2week(dat[1] + c(0, 15), week_start = 1, factor = TRUE)

If you already have an aweek object and want to convert it to a factor, you can use factor_aweek():


Be careful when combining factors with other dates or aweek objects as they will force the other objects to be truncated as well.

Weeks to weeks

You can use change_week_start() to convert between different week definitions if you have an aweek object:

w # week starting on Sunday
ww <- change_week_start(w, week_start = "wednesday") # same dates, starting on Wednesday
identical(as.Date(w), as.Date(ww))
# create a table with all days in the week
d   <- as.Date("2019-03-03") + 0:6
res <- lapply(weekdays(d), function(i) date2week(d, week_start = i))
names(res) <- weekdays(d)

All of these columns contain the same dates:

data.frame(lapply(res, as.Date))

Combining aweek objects

You can add dates, aweek objects, or characters to aweek objects:

  as.Date(res$Monday[1]) + 14)

However, you can not combine aweek objects with different week_start attributes.

c(res$Sunday[1], res$Wednesday[2], res$Friday[3])

If you want to combine different aweek objects, you must first change their week_start attribute:

wed <- change_week_start(res$Wednesday, get_week_start())
fri <- change_week_start(res$Friday, get_week_start())
c(res$Sunday[1], wed[2], fri[3])

Dates can be appended to aweek objects

Dates combined with aweek objects will will be automatically converted.

c(res$Monday, as.Date("2019-04-03"))

Add characters with caution

You can also add character representation of weeks, but be aware that it is assumed that these have the same week_start as the first object.

s <- c(res$Saturday, "2019-W14-3")
m <- c(res$Monday, "2019-W14-3")

These will translate into different dates


Working with weeks in data frames

You may encounter a situation where you have a merged data frame with weeks starting on different days. This section will cover two situations where you may have weeks as numbers and weeks as ISO-week strings. First we will create our demonstration data that represents the same week with different week_start attributes.

# create a table with all days in the week
d    <- as.Date("2019-03-03") + 0:6
res  <- lapply(weekdays(d), function(i) date2week(d, week_start = i))
resn <- lapply(weekdays(d), function(i) date2week(d, week_start = i, numeric = TRUE))
datf <- data.frame(wday = rep(weekdays(d), each = 7), 
                   week = unlist(res), # note: unlist converts to character
                   week_number = unlist(resn),
                   year  = 2019,
                   stringsAsFactors = FALSE)
datf$day <- substring(datf$week, 10, 11)
head(datf, 10)

To get the weeks (numbers or strings) to aweek objects, you should use the start argument to specify which day of the week they start on. Internally, this translates the week to their corresponding dates and then to aweek objects with the same week_start attribute (which defaults to get_week_start()).

weeks as numbers

Most commonly, you will have weeks across data sets represented by numbers. These can be converted to aweek objects using the get_aweek() function and to dates using the get_date() function:

datf$aweek <- with(datf, get_aweek(week = week_number, year = year, day = day, start = wday))
datf$date  <- with(datf, get_date(week = week_number, year = year, day = day, start = wday))
head(datf, 10)

These functions are also useful for constructing weeks or dates on the fly if you only have a week and a year:

get_aweek(11, 2019)
get_date(11, 2019)
datf$aweek <- NULL
datf$date  <- NULL

weeks as characters

If you have weeks formatted as ISO-week strings, then you can convert to aweek objects using as.aweek():

datf$aweek <- with(datf, as.aweek(week, start = wday))
head(datf, 10)

We can tabulate them to see how they transformed:

print(with(datf, table(before = week, after = aweek)), zero.print = ".")

Converting weeks to dates

If you receive data that contains week definitions, you can convert it back to a date if you know where the week starts.

week2date("2019-W10-1", week_start = "Sunday") # 2019-03-03
week2date("2019-W10-1", week_start = "Monday") # 2019-03-04

If you have an aweek object, however, it will use the week_start attribute defined in the object, even if the default week_start attribute is different:

set_week_start("Monday") # Set the default week_start to ISO week
get_week_start(w)        # show the default week_start for w
identical(week2date(w), dat)               # TRUE
identical(week2date(as.character(w)), dat) # FALSE

You can also use as.Date() and as.POISXlt() if you have an aweek object:


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aweek documentation built on Oct. 6, 2022, 9:07 a.m.