I'm convinced that conventional time travel - the sort that involves kicking DeLoreans up to 88 miles per hour - doesn't exist. If it did then everything in our day would be all messed up from the meddling, and someone from our era would have stolen the technology to use it for their own nefarious needs, for instance stealing a sports almanac to win enough money to build their own casino and demand Marty's mom get a boob job.
If I'm somehow wrong about this than I'm at least certain that time travel is only possible in an observational sense, and not participatory. Like watching a movie. Maybe the beings we think of as ghosts are really just time travelers who want to scare us. If so, thank you time travelers for making Halloween more fun.