Sundials normally measure “apparent solar time” and are unable to tell “clock time” because the analemma.  The earth’s tilt of 23.5 degrees and elliptical orbit result in a variability between the true position of the sun and the “mean” time that is regulated by clocks and watches.  A sundial can appear fast or slow compared to a clock by as much as 16 minutes depending upon the time of year.  To correct for this “equation of time”, Richard Schmoyer designed the gnomon of the Sunquest sundial with curves based upon the analemma.  His drawing of the gnomon design shows the precise calculations needed to make this correction.