This 3D Sundial simulates a real world sundial and displays local time so you can use it as a watch.
The sundial uses GPS and compass to determine location and orientation so it can calculate a custom 3D model of a sundial, tailered for your position and timezone.
The idea of using the compass is that the 3D view can align itself with the true north so that the simulated shadows match the real world shadows. If you know where true north is you can disable the compass for more accurate results.
In the 3D view you can tilt the phone and change the orientation of the phone. You can "zoom in" and "zoom out" within the 3D view using pinching. You can also change the 3D view a bit using draging. If you like you can try to find the sun in the 3D view.
A sundial will always show the solar time and not the time on your watch. The time the sundial indicates is off by a small amount, called the equation of time. To help you the current value of the equation of time is displayed in the top corner of the 3D view.