Transform Year/Week to Date Object

Transform year/week to date object

Before converting year-week to a date you have to specify a day of the week but more importantly you have to ensure which of the different conventions is being used.

Base R's strptime() function knows 3 definitions of week of the year (but supports only 2 of them on input) and 2 definitions of weekday number,
see ?strptime:

Week of the year

  • US convention: Week of the year as decimal number (00–53) using Sunday as the first day 1 of the week (and typically with the first Sunday of the year as day 1 of week 1): %U

  • UK convention: Week of the year as decimal number (00–53) using Monday as the first day of week (and typically with the first Monday of the year as day 1 of week 1): %W

  • ISO 8601 definition: Week of the year as decimal number (01–53) as defined in ISO 8601. If the week (starting on Monday) containing 1 January has four or more days in the new year, then it is considered week 1. Otherwise, it is the last week of the previous year, and the next week is week 1: %V which is accepted but ignored on input.

    Note that there is also a week-based year (%G and %g) which is to be used with %V as it may differ from the calendar year (%Y and %y).

Numeric weekday

  • Weekday as a decimal number (1–7, Monday is 1): %u
  • Weekday as decimal number (0–6, Sunday is 0): %w
  • Interestingly, there is no format for the case Sunday is counted as day 1 of the week.

Converting year-week-day with the different conventions

If we append day 1 to the string and use the different formats we do get

as.Date("2015101", "%Y%U%u")
# [1] "2015-03-09"
as.Date("2015101", "%Y%U%w")
# [1] "2015-03-09"
as.Date("2015101", "%Y%W%u")
# [1] "2015-03-09"
as.Date("2015101", "%Y%W%w")
# [1] "2015-03-09"
as.Date("2015101", "%G%V%u")
# [1] NA

For weekday formats %u and %w we do get the same result because day 1 is Monday in both conventions (but watch out when dealing with Sundays).

For 2015, the US and the UK definition for week of the year coincide but this is not true for all years, e.g., not for 2001, 2007, and 2018:

as.Date("2018101", "%Y%U%u")
#[1] "2018-03-12"
as.Date("2018101", "%Y%W%u")
#[1] "2018-03-05"

The ISO 8601 format specifiers aren't supported on input. Therefore, I had created the ISOweek package some years ago:

ISOweek::ISOweek2date("2015-W10-1")
#[1] "2015-03-02"

Edit: Using Thursday to associate a week with a month

As mentioned above you need to specify a day of the week to get a full calendar date. This is also required if the dates need to be aggregated by month later on.

If no weekday is specified and if the dates are supposed to be aggregated by month later on, you may take the Thursday of each week as reference day (following a suggestion by djhurio). This ensures that the whole week is assigned to the month to which the majority of the days of the week belong to.

For example, taking Sunday as reference day would return

ISOweek::ISOweek2date("2015-W09-7")
[1] "2015-03-01"

which consequently would associate the whole week to the month of March although only one day of the week belongs to March while the other 6 days belong to February. Taking Thursday as reference day will return a date in February:

ISOweek::ISOweek2date("2015-W09-4")
[1] "2015-02-26"

How do I transform week (and year) to date of the first date (i.e. Monday) of the week in R?

Your code is good. But you have only one problem: NA if week == 0. If week number is zero then first day of the year is not monday. So you can check if week number is zero (and find monday in previous year) or do as in you example. For example as.Date("2018-1-1", "%Y-%W-%w") == 2018-01-01 (week == 1, not 0).

foo <- function(year, week) {
if (week == 0) {
year <- year - 1
week <- data.table::week(as.Date(paste0(year, "-12-31"))) - 1
}
return(as.Date(paste0(c(year, week, "1"), collapse = "-"), "%Y-%W-%w"))
}

Convert week number to date

as.Date is calling the 1 to 9 as NA as it is expects two digits for the week number and can't properly parse it.

To fix it, add in some - to split things up:

as.Date(paste(2014, df$Week, 1, sep="-"), "%Y-%U-%u")

How to convert week numbers into date format using R

Maybe there is a more automated way, but try something like this. I think this gets the right days, I looked at a 2020 calendar and counted. But if something is off, its a matter of playing with the (week - 1) * 7 - 1 component to return what you want.

This just grabs the first day of the year, adds x weeks worth of days, and then uses ceiling_date() to find the next Sunday.

library(dplyr)
library(tidyr)
library(lubridate)

df %>%
separate(week, c("year", "week"), sep = 4, convert = TRUE) %>%
mutate(date = ceiling_date(ymd(paste(year, "01", "01", sep = "-")) +
(week - 1) * 7 - 1, "week", week_start = 7))

# # A tibble: 6 x 4
# year week Revenue date
# <int> <int> <dbl> <date>
# 1 2020 9 4543 2020-03-01
# 2 2020 10 6764 2020-03-08
# 3 2020 11 2324 2020-03-15
# 4 2020 12 5674 2020-03-22
# 5 2020 13 2232 2020-03-29
# 6 2020 14 2323 2020-04-05

How can I convert year and week to Java Date object?

Solution using standard java classes. This just gets the current time and sets the fields of year and week of year to known values.

Calendar cld = Calendar.getInstance();
cld.set(Calendar.YEAR, year);
cld.set(Calendar.WEEK_OF_YEAR, week);
Date result = cld.getTime();

Convert year.week to date

If you append a weekday then as.Date will work:

x <- c("2016.49", "2016.50", "2016.51")

as.Date(paste0(x, ".1"), "%Y.%U.%u")

[1] "2016-12-05" "2016-12-12" "2016-12-19"

Convert Week number to date in data frame

You can do this in base R using as.Date itself.

Based on your attempt it seems your locale is English, so you can try :

as.Date(paste(df$Week, df$Year, 'Sun'), '%U %Y %a')
#[1] "2019-01-06" "2019-01-13" "2020-01-05" "2020-01-05"


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