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

Answer 1

The issue is because for %u, 1 is Monday and 7 is Sunday of the week. The problem is further complicated by the fact that %U assumes week begins on Sunday.

For the given input and expected behavior of format = "%Y-%U-%u", the output of line 4 is consistent with the output of previous 3 lines.

That is, if you want to use format = "%Y-%U-%u", you should pre-process your input. In this case, the fourth line would have to be as.Date("2016-51-7", format = "%Y-%U-%u") as revealed by

format(as.Date("2016-12-18"), "%Y-%U-%u")
# "2016-51-7"

Instead, you are currently passing "2016-50-7".

Better way of doing it might be to use the approach suggested in Uwe Block's answer. Since you are happy with "2016-50-4" being transformed to "2016-12-15", I suspect in your raw data, Monday is counted as 1 too. You could also create a custom function that changes the value of %U to count the week number as if week begins on Monday so that the output is as you expected.

#Function to change value of %U so that the week begins on Monday
pre_process = function(x, delim = "-"){
    y = unlist(strsplit(x,delim))
    # If the last day of the year is 7 (Sunday for %u),
    # add 1 to the week to make it the week 00 of the next year
    # I think there might be a better solution for this
    if (y[2] == "53" & y[3] == "7"){
        x = paste(as.integer(y[1])+1,"00",y[3],sep = delim)
    } else if (y[3] == "7"){
    # If the day is 7 (Sunday for %u), add 1 to the week 
        x = paste(y[1],as.integer(y[2])+1,y[3],sep = delim)
    }
    return(x)
}

And usage would be

as.Date(pre_process("2016-50-7"), format = "%Y-%U-%u")
# [1] "2016-12-18"

I'm not quite sure how to handle when the year ends on a Sunday.