I just became aware of modern sundials, which should be as precise as a modern clock / watch through alt.horology:

http://atensundials.com/

Can anyone appraise the statements made and give me some explanation on how it works, use and reliability with less than medium sun exposure.

Appreciating your comments.
Your Swiss Watch Friend

The statement about the precision of the sundial is right: the sun follows a very precise path that can be related to our "time" by mathematical equations. These equations are a function of the position of the site (longitude, latitude) and time of the year.

The sundials have been constructed such that the geometrical arrangement of the parts can represent these equations. The time precision is given by the mechanical precision of the parts and by the precision in positioning the sundial.

For the sundial to be operative you need of course a defined shadow produced by the rays coming directly from the sun onto the sundial, so a cloudy day is no timed at all...