What time is
Time is a scalar quality.
The frequency of actions you make both physically and mentally make future actions of that kind easier. Like a river it erodes away the obstacles. It's using the same pressure gradient over and over again. This method works over the whole spectrum of interactions. Start with doing something very small then keep doing that. Time is a unit of change. A 3rd factor determines how much reactions happen in how much time. Say we have an area where time is slow allowing you to do more things per unit of time. It would take the most impossible engineering efforts and you won't even be aware of having more time.
Local time travel within the brain is much more desirable. It's important not to make effort of it. Such conditioning requires upkeep which demands time. The thought soup needs to be accelerated slowly. There just have to be time invariant systems in there, we want as much of those effects as we can get. Slowly, all of the scalar trains of thought need to stay in the rails.
So, I guess the closest thing to useful time travel we have is broccoli, carrots and apricot seeds. Very soon we will be able to transfer peoples thoughts to a computer. Combined with quantum computing and VR technologies it will make for a very life like simulation. Still the number of thoughts you have per unit of time depends on your own choice of wiring.
Thus, time is clearly something we've made up. ^_^