It appears that either answer would be accepted, and so I'm fine with it. If it really is there is one correct answer then I'm against this. This feels like a problem where a good enough solution can be done in the time of an interview if you do it by hand (though if anyone knows about dates they will expect there is a lifetime of fixing special cases left if you don't use the library)

I prefer fizz-buzz as a question because it is obvious there isn't a library. It is also a problem you should be able to do in an interview. It has enough weirdness that there is no best answer, despite having several workable paths you could try.