Louis Tavernier (1754-1840) was the son of the renowned watchmaker Jean-Louis Tavernier and worked closely with Abraham-Louis Breguet. The dials are stamped Lucard, a maker of enamel dials in Paris.
Tavernier built the calendar indication of the present watch on the difference between "Sun Time" and "Clock Time" and indicates the months when the sun is "late" (retarde) and when it "increases" (avance).
"Sun Time" is taken from the idea that it is noon when the sun reaches its highest point or crosses the meridian and it will be noon again the next day when it crosses again the meridian. The elapsed time between successive noons is sometimes more and sometimes less than 24 hours of clock time. In the middle months of the year, the length of the day is close to 24 hours, but around 15 September the days are only 23 hours, 59 minutes and 40 seconds long while around Christmas, the days are 24 hours and 20 seconds long.
"Clock time" is based on the assumption that a day is exactly 24 hours long. Although this is not totally true it is obviously much more convenient to have a "mean sun" which takes exactly 24 hours for each day, since it means that mechanical clocks and watches can be made to measure these equal time intervals.
The small differences in the lengths of "sun days" and "mean days" add up to larger differences between "sun time" and "clock time". A peak of slightly over 14 minutes is reached in mid-February, when "sun time" is slow compared to "clock time" and just over 16 minutes at the beginning of November when "sun time" is fast compared to "clock time".
The equation of time is basically a mathematical division by adding to or subtracting from sun time the amount necessary to obtain mean or clock time.