Ancients had no zero digit. They counted time using strictly 1 based counting numbers. Modern Astronomers have introduced a year zero between 1 BC and 1 AD that historians would not have acknowledged. This introduces errors.
The number zero was not understood as being important until about the year 600 AD. Thus zero was not a conceivable year number on any calendar in use before this time.
For proof of this go find a corner stone on a building errected before 1900. Dates were almost exclusively written with Roman Numerals. Consider that even in the United States, most dates were recorded using Roman Numerals, even into the 1900s. This shows up on copyright dates in holywood movies even today. Book copyrights from the 1800s are almost exclusively done with Roman numerals. In the Roman Numeral System there is no numeral for zero. There was no way that anyone using this system could have ever expressed a year zero, since it was not possible to even write such a thing using the common notation for dates that was not abandoned until this century.
Historians thus never considered year 1 as being preceded by year zero, they considered year 1 AD as being preceded by year 1 BC. They had no choice, they could write it no other way. That 1 BC came before 1 AD made perfect sense to everyone through at least the 600s with the introduction of the Arabic number system used today. Even then, dates were still commonly recorded with Roman Numerals into the 1900s.
Because the first century started with the year 1, historians consider all centuries to start on year 1 of that century, the first running 1 through 100, the 20th running 1901 through 2000. Centuries before the year 1 run from the 100’th year through to the 1’th year, making it similar to the century structure used by astronomers.
Calculating differences in years is error prone across the year 1 since correction must always be made for the missing year zero. The online system here focuses more on counting day-differences and through the use of a day-accurate epoch at Adam, it has no problems with any date recorded anywhere by mankind.
In this database the Gregorian calendar is rendered with year zero, the Julian is not. This allows dates rendered on the Gregorian using astronomical techniques to be mapped across. BE AWARE that well meaning astronomers may already have corrected for the missing year zero. If they have done so, you risk Drift Error and must carefully determine if you should correct back, and insert a year zero before looking up the Gregorian date. This drift problem is so pervasive that no date given as B.C. should be trusted with any accuracy.