This app is based on the XKCD Phone comic ( http://xkcd.com/1363/). It allows you to set an alternative speed of light and see the effects of time dilation as you approach it. The app clock can be selected to pass time either at the rate time appears to pass in the surroundings to an observer traveling at speed or at the rate time appears to pass for an object at speed to a stationary observer.
NOTE: This app is not affiliated with nor endorsed by XKCD or Randall Munroe. Use of the name XKCD is under the Creative Commons license (found at http://creativecommons.org/licenses/by-nc/2.5/)