as.Date produces unexpected result in a sequence of week-based dates

Question

I am working on the transformation of week based dates to month based dates.

When checking my work, I found the following problem in my data which is the result of a simple call to as.Date()

as.Date("2016-50-4", format = "%Y-%U-%u")
as.Date("2016-50-5", format = "%Y-%U-%u")
as.Date("2016-50-6", format = "%Y-%U-%u")
as.Date("2016-50-7", format = "%Y-%U-%u") # this is the problem

The previous code yields correct date for the first 3 lines:

"2016-12-15"
"2016-12-16"
"2016-12-17"  

The last line of code however, goes back 1 week:

 "2016-12-11"

Can anybody explain what is happening here?

Answer 1

Working with week of the year can become very tricky. You may try to convert the dates using the ISOweek package:

# create date strings in the format given by the OP
wd <- c("2016-50-4","2016-50-5","2016-50-6","2016-50-7", "2016-51-1", "2016-52-7")
# convert to "normal" dates
ISOweek::ISOweek2date(stringr::str_replace(wd, "-", "-W"))

The result

#[1] "2016-12-15" "2016-12-16" "2016-12-17" "2016-12-18" "2016-12-19" "2017-01-01"

is of class Date.

Note that the ISO week-based date format is yyyy-Www-d with a capital W preceeding the week number. This is required to distinguish it from the standard month-based date format yyyy-mm-dd.

So, in order to convert the date strings provided by the OP using ISOweek2date() it is necessary to insert a W after the first hyphen which is accomplished by replacing the first - by -W in each string.

Also note that ISO weeks start on Monday and the days of the week are numbered 1 to 7. The year which belongs to an ISO week may differ from the calendar year. This can be seen from the sample dates above where the week-based date 2016-W52-7 is converted to 2017-01-01.

About the ISOweek package

Back in 2011, the %G, %g, %u, and %V format specifications weren't available to strptime() in the Windows version of R. This was annoying as I had to prepare weekly reports including week-on-week comparisons. I spent hours to find a solution for dealing with ISO weeks, ISO weekdays, and ISO years. Finally, I ended up creating the ISOweek package and publishing it on CRAN. Today, the package still has its merits as the aforementioned formats are ignored on input (see ?strptime for details).