Of course, the Julian calendar is available in Emacs, as are the Hebrew, Chinese, etc.
Calc and the Emacs calendar use different calendar systems.
From the Calc manual:
Calc uses a combination of the Gregorian and Julian calendars,
following the history of Great Britain and the British colonies.
This is the same calendar that is used by the `cal' program in most
Unix implementations.
and from the Emacs manual:
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.
So, for example, the day before September 14, 1752 is
September 2, 1752 according to Calc and September 13, 1752 according
to the calendar.
Is this acceptable, or should they be made consistent?
Jay