In Justice League (the animated series which ran from 2001 to 2004) season 1 finale the "The Savage Time", Justice League members use a time machine built by Vandal Savage to return to World War 2 to prevent the Nazis from defeating the Allies and having the world history be changed.
My question is how did they manage to travel back to the present time once they fix history back to how it was supposed to be? This never seems to have been explained and I don't believe any of them had the knowledge to construct a time machine, especially with the tech back then. And how did Batman know to expect them in the Watch Tower for their return? I don't think the Watch Tower was equipped with a time machine so a black hole popping out of existence would surely have freaked him out, right?