A while back a guy claiming to come from the future, John Titor, found a following on the Internet. Any future predictions were hazy, but if anything he said failed to pass he had an out: It's possible he came from a different future than the one we will experience.
I'm sorry to be skeptical, but even if it were possible to build a time machine you couldn't just calculate the time you want to go to; you also have to figure out where in space you want to go.
You can't just return to the same spot. The spot on the planet you are starting at won't be in the same place in the time you are going to. First off, the world is spinning at about 1,000 miles per hour, depending on your latitude, so even going five minutes into the past won't get you to the same place. On top of that, it is circling the sun at about 62,000 mph.
Further, our solar system is circling the Milky Way Galaxy at 155 miles per second and the Milky Way is moving in our Local Group of galaxies at 185 miles/sec. Worst of all, we don't know exactly how fast the universe is expanding. It's about 71 km/s/Mpc, but that's only within a 5 percent margin of error. And you have to be exact on every single calculation to wind up in the "same spot" on the earth. (We didn't even discuss the wobble of the earth.)
Before you tell me I've forgotten relativity, I haven't. Maybe it's just like tossing a quarter up and catching it while you're on an airplane. The quarter travels with you as though you were standing still. Problem is, the earth experiences periods of irregular rotation which can't be predicted.
Still, for sake of argument, let's say you had all these calculations perfectly precise. You still have to know there isn't a person, animal or any other object in the spot you intend to pop into. (For that matter there has to be something there -- even if it's air. You and your machine cannot occupy the same space as air molecules any more than you can occupy the same space as a brick wall.)
But beyond that, if a time machine had ever been built someone would have already come back without being so secretive as John Piper. Once that ship has sailed it would be just like the atomic bomb. Everybody and his brother would have access to one eventually. There'd be a ride in Disney World.
Eventually somebody will use it for evil -- lots of somebodies, in fact. And every somebody who uses it for "good" will try to stop Hitler, John Wilkes Booth, Lee Harvey Oswald, etc. Yet nobody ever succeeded in going back in time and killing Hitler. If they had, you wouldn't be reading this.
There is no time travel of the type we see in movies and never will be. Give it up.