Not an issue, 1.6K ppl completed the kata in JS

Resolved.

The kata creator is sloppy and the display message actually uses May 13 instead of May 3 (which is what was being tested), hence all the confusion.

i.e Your code is wrong because you didn't account for 1 digit days.

This has happened to me before and may be a server issue or small bug somewhere.

Try printing the input in the function, as well as your output.
And see if you can figure out when&why your function is returning the wrong result.
Doing the problem by hand for the failing values might speed up the search.
If you are still stuck, post your code here.

Might be a whitespace problem(browsers hide those).
Make sure there are no extra spaces at the start or end of your output string.
If you still face the same problem, post your code here(mark it as spoiler).

Please, change status for issue as resolved.

Test works fine. It is not necessary to post actual solution here, just describe in details your issue

What language?

Show your solution. My both JS and Python solutions have been successfully accepted just now.