ok.. I'll only say it once so pay attention
1st: intelligent aliens may or may not exist. but they certainly haven't been into the solar system.
2nd: dinosaurs weren't killed off by a meteor. the meteor strike set a bottleneck event causing a lot of extinctions, but dinosaurs prevailed.. as birds. moreover, fossils from extinct dinosaurs were found and dated after the meteor strike.
3rd: meteor strikes happen all the time... just because one was large enough it doesn't mean it had to be a weapon.. every 500 million years, one of those should kick in by chance.
4th: there's no base in the moon.. that just doesn't make sense. the moon has no life-support resources and it is waay to expensive by energy investments (something that not even aliens may escape from) to bring them from a home world.. it would make more sense that they made one in the earth itself..
5th: so they striked.. then what?! wouldn't they.. i don't know, have taken the earth after that?
6th: although the strike had a point in time, it took hundreds of thousands of years to kill off most species in that extinction event.. so even if aliens needed to get rid of the dinosaurs.. they would have looked for a more immediate solution.
7th: how the F*** are we supposed to know? we would need to date down the strike in at least the exact day.. that's beyond our methods.
8th: sometimes, there aren't real dangers out there.. don't invent them and become a paranoid delusional.