I understand that present perfect can be used with "today" if "today" isn't finished. So if I say: "I've seen him today" it is still today and the fact that I've seen him is still relevant at the time I'm saying it.
Sometimes people say you can only use present perfect with "today" if there is a possibility the action, in this case seeing him again, can be repeated the same day. That is what I don't understand, why is it important if I see him again that day? If I have seen the person once that day and it is relevant at 10 p.m. can I still say: "I've seen him today"? The day is still unfinished at 10 p.m , whether I see him again or not.