Depends, as long as the mage in question can't actually reverse time, you could probably make a machine that locally accelerates or stop time, going backwards would require overcoming paradoxes that reverses the time travel process itself and renders it ineffective.
It would seem that if you built a skyscraper sized complex, supported by air and earth magic, and then established an entire water cycle (self contained electrical heating powering turbines that keeps the electricity flowing, with the heat driving the water to the top and it condenses, powering other turbines)in it, you could get a hell of a lotta power, and the means to direct it. Well, technically, store a lot of power, it'd be like a giant magic capacitor.
You could have citizens required to pay their taxes in energy. Heck, base the entire economy on that device, it provides all the work and power you'd ever need in exchange for a tithe of power from every one. Not to mention in times of war, it'd make one hell of a magic laser.