It is hard to tell, at that. I've had similar problems not because of time period, necessarily, but because the people of Midgard aren't all that well traveled. So is it that Asgard does not exist in their iteration of the universe, or just that they haven't been there yet?

Similarly I wonder if it's truly that the Aesir aren't about in your time period - thus different universes - or just that they've finally destroyed themselves with stupidity or, less likely, ascended beyond their normal state.