Two different calendar systems are used, Gregorian and Julian.
       These are nearly identical systems with Gregorian making a small
       adjustment to the frequency of leap years; this facilitates
       improved synchronization with solar events like the equinoxes.
       The Gregorian calendar reform was introduced in 1582, but its
       adoption continued up to 1923. By default cal uses the adoption
       date of 3 Sept 1752. From that date forward the Gregorian
       calendar is displayed; previous dates use the Julian calendar
       system. 11 days were removed at the time of adoption to bring the
       calendar in sync with solar events. So Sept 1752 has a mix of
       Julian and Gregorian dates by which the 2nd is followed by the
       14th (the 3rd through the 13th are absent).