If it is a first date, you don't have to say anything. If the date goes well and you see each other again, get the feel of your date's thoughts about various subjects. You may be able to touch on the topic of mental health without saying anything outright about your dx. If it looks like a relationship is forming, then get some advice from a therapist on how to approach the subject. If the date does not go well, you just part, and there is no need to talk about your mental health status.