Sundials using an indicator parallel to the earth's axis and straight hour lines, as we know them e.g. from walls of buildings, deviate by about 16 minutes from the standard time which shown by our watches. This deviation is called the Equation of Time. The reason for that is the inclination of the earth's axis against the ecliptic and the elliptic orbit of the earth around the sun.
Already in the year 1866 General Oliver had the idea to shape the indicator such that by help of the daily changing inclination of the sun the equation of time is corrected. A well known realization of this idea is Bernhard's sundial. Since however the sun shows twice a year the same declination but the equation of time is different for these two dates, this indicator has to be changes twice a year. That's why the realization of this dial is usually quite small.
The main feature of this novel dial is the idea to develop a double indicator which avoids changing the indicator twice a year and therefore represents the full equation of time. Since the equation of time is a unique function of the inclination of the earth's axis together with the earth's elliptic orbit, the double indicator represents Kepler's first and second law.
Another special feature of the sundial is the indication of the date with an accuracy of ±1 day, by using the shadow which the disk casts on the indicator.