A note on http://www.hineslab.com/digital-sundial/ linked elsewhere in the thread states you can get +/- 2 minutes accuracy with the sun.

I wonder where he gets that from. Due to the equation of time, the difference can be up to about 15 minutes. But maybe he compensates for that already in some way? https://en.wikipedia.org/wiki/Equation_of_time (This is of course ignoring things like Summer Time.)