I am guessing the problem arises from the use of Locale.
The locale string might be different for your place while different for the codewars platform.

In either case, a simple console.log(date) after the reassignment of date will show you the problem.

Hard to say without seeing your code.
But a common mistake on this kata is to mistake the date object for a string.

If that's not the case with you, just post your code here(mark it as spoiler).