You'll be pleased to learn that there are at least two known solutions to General Relativity that allow time travel. The first requires the universe to be rotating (at present, it isn't) so that you can send your spaceship off in a particular direction, you'll travel in time. So all you need to do is go back in your time machine and tinker with the big bang so that we end up with a rotating universe. Simples! But I can't help but feel I've missed something obvious here...
The other is to get a rotating cylinder of neutronium (the same material that neutron stars are made of.. and, @
chriss2.0 neutron stars and black holes are very well described by General Relativity): that generates areas where time goes backwards. So all you need to do is take the sun, compress it into a cylinder 10 km in diameter and 50 km long and spin it so that its surface is travelling at close to the speed of light. You're probably not going to be able to fit that into the garden shed, though. And you may get people complaining about the lack of sunlight, lack of crops and the oceans and atmosphere freezing. You know, small pesky inconsequential details and stuff. Just tell them that they're luddites who're standing in the way of Progress, that'll sort them.