* calc/calc.el (calc-gregorian-switch): New variable.

* calc/calc.el (calc-gregorian-switch): New variable.

* calc/calc-forms.el (math-day-in-year, math-dt-before-p)
(math-absolute-from-gregorian-dt, math-absolute-from-julian-dt)
(math-date-to-julian-dt, math-date-to-gregorian-dt): New functions.
(math-leap-year-p): Add option to distinguish between Julian
and Gregorian calendars.
(math-day-number): Use `math-day-in-year' to do the computations.
(math-absolute-from-dt): Rename from `math-absolute-from-date'.
Use `math-absolute-from-gregorian' and `math-absolute-from-julian'
to do the computations.
(math-date-to-dt): Use `math-date-to-julian-dt' and `math-date-to-gregorian-dt'
to do the computations.
(calcFunc-weekday, math-format-date-part): Use the new version of the DATE to
determine the weekday.
(calcFunc-newmonth, calcFunc-newyear): Use `calc-gregorian-switch' when necessary.