It doesn't need to do anything more than the existing app - just less:
Currently, to track time when away from your desk you need to:
  1. touch the app icon
  2. touch plus
  3. touch track time
  4. [most likely] scroll down to the task of choice [because its a really short list taking around ⅓ of the screen]
  5. touch the task you want.
With a dedicated app, items 2-3 and a good amount of 4 would be removed from the equation - a massive time-saver if you do this frequently. It will also reduce the overhead that makes it less likely that you track time.