"Which date is it today?"
Translation:Vilket datum är det i dag?
But the question was specifically "Which" not "What" so Vad är dagens datum? would not be right in this case, right? (FYI: Saying "Which date is it today?" sounds odd in English. Saying "What is today's date?" makes more sense. Although, most people, at least in Canada and the US, would say "What is the date today?")
I'm a teacher and most students in Massachusetts would say, "What is today's date?"
= "Which is the day's date" - sounds odd in English too.
Also, 'dagen' is an en-word, so if your pointing to that word, [which] should be [vilken].
It should accept both all the time, but if you see that "idag" is missing somewhere, you should report it.
på can be used when asking date for a day. 'Vilket datum är det på onsdag?' i dag means today and på cannot be added here.