calc-gregorian-switch

calc-gregorian-switch is a variable defined in `calc.el'.
Its value is

nil

Documentation:
The first day the Gregorian calendar is used by Calc's date forms.
This is `nil' (the default) if the Gregorian calendar is the only one used.
Otherwise, it should be a list `(YEAR MONTH DAY)' when Calc begins to use
the Gregorian calendar; Calc will use the Julian calendar for earlier dates.
The dates in which different regions of the world began to use the
Gregorian calendar vary quite a bit, even within a single country.
If you want Calc's date forms to switch between the Julian and
Gregorian calendar, you can specify the date or choose from several
common choices. Some of these choices should be taken with a grain
of salt; for example different parts of France changed calendars at
different times, and Sweden's change to the Gregorian calendar was
complicated. Also, the boundaries of the countries were different at
the times of the calendar changes than they are now.
The Vatican decided that the Gregorian calendar should take effect
on 15 October 1582 (Gregorian), and many Catholic countries made
the change then. Great Britain and its colonies had the Gregorian
calendar take effect on 14 September 1752 (Gregorian); this includes
the United States.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 24.4 of Emacs.