There was an earlier thread about posting B.C.E. dates.
The key was putting a negative number in the date editor.
I did an additional test using the year -100000000000. The resulting entry was 9999 B.C.E. Obviously, there is a limit.
With another entry 6666 B.C.E. (entered with the date editor) and then the events sorted by date, 6666 B.C.E. was sorted first before 9999 B.C.E.