The Emacs calendar displayed is always the Gregorian calendar, sometimes called the New Style calendar, which is used in most of the world today. However, this calendar did not exist before the sixteenth century and was not widely used before the eighteenth century; it did not fully displace the Julian calendar and gain universal acceptance until the early twentieth century. The Emacs calendar can display any month since January, year 1 of the current era, but the calendar displayed is always the Gregorian, even for a date at which the Gregorian calendar did not exist.
While Emacs cannot display other calendars, it can convert dates to and from several other calendars.