According to Duo "thirty-first" is "almost correct" and "thirty_first" is correct. I've reported it.
Duolingo ignores punctuation which is how it considers 'thirty_first' is marked as correct even though it's not listed as a correct option, 'thirty-first' is not listed as a correct option.
Right - Duo ignores punctuation and because of that I usually do too, but for some reason I felt like using the hyphen. Thanks for the explanation!
Actually I mistyped, what I meant was that we've added thirty-first as a correct option now, thanks for the original comment.