Among other things, Einstein's theory of special relativity says that as an object's velocity increases, time as experienced by the object will slow down when compared to another object traveling at a lower velocity. This means that a "relatively" short round trip on a space ship traveling at close to the speed of light would see you arrive home having aged less than those back on Earth. While the greater the velocities involved, the greater the effect, the theory applies to all relative movement. Now there's an iPhone app that will let you know just how many extra nanoseconds you've gained by getting moving as opposed to sitting on your rear end.
To calculate just how much time you've gained by walking to the shops, the Einstein's Pedometer app uses the iPhone's GPS capabilities and the Lorentz transformation, which describes how two observers' varying measurements of space and time can be converted into each others frame of reference. A quick stroll round the neighborhood with Einstein's Pedometer yielded me an extra 0.00021440 nanosconds than if I'd stayed at my desk doing something else, like working. I plan to spend the extra time composing my memoirs.