There are other day number systems more common than the  emacs
"absolute" day number --
the "astro" or Julian day number and the proleptic Gregorian system
used by sqlite3
e.g. select julianday("-4713-11-24").
cal-julian.el:alendar-astro-to-absolute refers to  "astro".
Proleptic Gregorian day 0 = November 24, 4714 BC while "astro" day 0 =
 January 1, 4713 BC.
I'm attaching julian.lisp which has calendar-correlate-from-date and
calendar-correlate-from-julianday
may help with days before 1582. These are based on Peter
Duffett-Smiths calculations.