calendar-absolute-from-gregorian is a compiled Lisp function in `calendar.el'.

(calendar-absolute-from-gregorian DATE)

The number of days elapsed between the Gregorian date 12/31/1 BC and DATE.
The Gregorian date Sunday, December 31, 1 BC is imaginary.
DATE is a list of the form (month day year). A negative year is
interpreted as BC; -1 being 1 BC, and so on. Dates before 12/31/1 BC
return negative results.