> I like the idea of solving this by just asking for the location of an event.

One issue with this phrasing is that a user might react--quite reasonably--with frustration like: "Gawd, stop asking me you stupid computer, it doesn't matter, I already told you it's an online-only teleconference, it's not really anywhere!"

I'm not sure about the best way to phrase it, but ideally we want the user to be answering: "Which clocks determine when the meeting starts?"