How to get week numbers from dates?

Looking for a function in R to convert dates into week numbers (of year) I went for week from package data.table. However, I observed some strange behaviour:

> week("2014-03-16") # Sun, expecting 11
[1] 11
> week("2014-03-17") # Mon, expecting 12
[1] 11
> week("2014-03-18") # Tue, expecting 12
[1] 12

Why is the week number switching to 12 on tuesday, instead of monday? What am I missing? (Timezone should be irrelevant as there are just dates?!)

Other suggestions for (base) R functions are appreciated as well.

Solution 1:

Base package

Using the function strftime passing the argument %V to obtain the week of the year as decimal number (01–53) as defined in ISO 8601. (More details in the documentarion: ?strftime)

strftime(c("2014-03-16", "2014-03-17","2014-03-18", "2014-01-01"), format = "%V")

Output:

[1] "11" "12" "12" "01"

Solution 2:

if you try with lubridate:

library(lubridate)
lubridate::week(ymd("2014-03-16", "2014-03-17","2014-03-18", '2014-01-01'))

[1] 11 11 12  1

The pattern is the same. Try isoweek

lubridate::isoweek(ymd("2014-03-16", "2014-03-17","2014-03-18", '2014-01-01'))
[1] 11 12 12  1