Michael, I can't speak for the correctness of that code, but you or that code may be confusing some things.
There's the proleptic Gregorian calendar, and there's the proleptic Julian calendar. They differ by about a month when you go back to near the Julian epoch.
Then there's the way ISO years are counted, and the way Gregorian/Julian years are counted. The former have a zero, and go negative. The latter don't have a zero, and don't go negative, but rather annotate with BC or whatever.
In any case, unless you are using MATLAB pre-R2-14b, just use datetime:
>> datetime(0,'convertfrom','juliandate','Format','dd-MMM-yyyy G HH:mm:ss')
24-Nov-4714 BCE 12:00:00
Those are in the proleptic Gregorian calendar with ISO and Gregorian year conventions, respectively.