This perpetual calendar works for any year since 4713AC (-4712). The date of change from Julian Calendar to Gregorian is
Oct 15 1582 (Julian Oct 5 1582).
For years before 321, when Emperor Constantine I introduced by decree the week cycle and
Sunday rest, you will see the Roman Calendar with Kalendae, Nonae and Idi and the nundinal cycle
instead of the week. You can always see the modern style calendar selecting from
the dropdown menu.
For years between 46BC (first year of the Julian Calendar)
and 4AD I tried to take into account the leap year error (which the priests interposed every three years instead of four) up to 8BC when August corrected this error suppressing three leap years.