sorry, wrong testcase. @deconstrained already had the right hint

Hey there,

at least for Iso 8601 the first calender week does not start on the first monday in january, but instead in the week containing the first thursday in january.
Every calendar week starts with a monday, and thus has always a duration of seven days.
Please consider taking that in your Description.

Alternatives like in USA (at least believing the german wikipedia entry) start exactly at the first of january and result in the first week being shorter than seven days.

If you'd like to set the scenario to "starting at first monday" consider adding the word: "asume".

In my opinion, the javascript tests are furthermore failing to check for illegal arguments like any "NaN".
Either have them checked or exclude them in the task description.