I'm gonna start putting that comment at the top of solutions to feel better about myself

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

Yes, the first sentence in the description was wrong. Sorry for that. This is now fixed and an image was added to outline this.

You are absolutely right. I adjusted the description to describe the ISO 8601 specification.

I think the description was wrong because the wiki says that whatever year the Thursday is in tells what year the week counts as. So on that one the thursday is in 2019 so the first Monday is the start of week 2.

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.