I agree with the other planet thing. Another thought...how about another dimension? Say Tempus traps Lois and Clark on an Earth where humanity never evolved/appeared or all died off?
Depending on what you want from the story, you could have a "Life after earth" scenario, where humanity vanished over one day and all that's left is a spooky, uninhabited world of ruined buildings, or an earth that was never inhabited, for them to roam.
Of course, it might be embarrassing for them to run around nude in the nfic version when HG Wells finally shows up to save them...
