I wasn’t trying to update my birthday which obviously remains unchanged. A prompt on Trademe on my phone this morning ‘asked me’ to check and update my details, which I did. Until I got to the birthday field, which didn’t work at all. I don’t get it, so never mind and thanks for trying to help. Maybe Trademe can clarify why I even received the prompt in the first place and secondly why the birthday field is not working.

Thanks everyone, I'll ask Trademe to remove the question.