hadley/lubridate — pull request #197 by jasonelaw (2013-08-14 20:24:12+00:00)
Title: ISO 8601 week extractor

I added a an `isoweek` function which returns the [ISO 8601 week](http://en.wikipedia.org/wiki/Week_number#Week_numbering). It does what `format(x, '%V')` does on systems that support that conversion specification. Since I'm on windows, I was out of luck. The code is adapted from http://en.wikipedia.org/wiki/Talk:ISO_week_date#Algorithms.