GNU Emacs Manual
AC.9 Conversion To and From Other Calendars
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 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.
(aside from Gregorian).