Now
Hi Nick -
It was an airburst, so I'm surprised there's any crater at all.
How do we KNOW it was an airburst? From the remains excavated nearby, the product of an airburst. Of course, you could always propose an ancient nuclear war, and the "fringe" in the US does that all the time. It's an article of faith for many of them.
As far as dating goes, the excavtrion report only gives us a rough date, as the site was excavated long ago.