This calendar works starting Jan 1 -4712(4713AC) which is Julian day 0.
It uses julian calendar up to Oct 4 1582, gregorian calendar hereafter (from Oct 15 1582).
For years before -45 (46AC) when Julius Caesar introduced his calendar, dates follow a retroactive Julian Calendar, not the Numan calendar which was then in use (there is too little information to rebuild it in detail)
Between -45 (46AC) e 8DC I tried to take into account the changing of name and length of the months and the mistake in computing leap years (one in three years instead of four) made by the Romans until 8BC.