It actually makes more sense what Mick noted than "aliens" ...
If you think about it we already have some rudimentary time travel...
We know for a fact that the faster an object moves the slower the relative time of that object is.
We had to tweek the GPS system for example to make up for the time dilation of a few milliseconds due to the fact that the orbiting satellites were reporting a slightly slower time.. In other words we have a basic understanding of how to slow down time.. hence travel forward in time.
Theoretically we also "know" how to go backwards in time as well... we just have the exceed the speed of light... something that is well beyond our understanding of physics currently.
So you want a time machine that allows you to go forward and backwards in time? It's simple.
Speed yourself up to 99.99% the speed of light... and you'll age super slow and can travel forward in time... every second you're on that thing 100 years would pass on the ground.... want to go backwards in time... go 200% the speed of light... every second you're on that thing you'll go backwards 100 years....
The problem is nothing can go faster than light to go backwards in time.... and to go 99.99% the speed of light would require the energy output of the sun..... and you guys complain about the price of oil
Of course it wasn't Bush's fault, the mission was accomplished.