hi! Not discouraging other answers here.... while the other respondents are attempting to address the question of "year 0", if you're interested in general information on the development of the year numbering system, there's more in the FAQ:

• How did the world agree on what year it is?

Meanwhile, if the adoption of "zero" is the core question, you can get started on these earlier related posts:

• comment in AMA History of Science

• comment in Is there a chance that before its destruction the library of Alexandria contained information that we still don't know today?

• comment in Why is the area of Roman mathematics not as well known as other comparable civilizations, e.g. Greek or Egyptian math?

• thread in Were there any scientific discoveries in the Americas that were unknown in the Old World?

• Did, and how did, cultures without a mathematical zero understand and represent the concept of "nothing?"

• How did mathematics work before the introduction of the number zero?

• There is no Roman numeral for the number zero. How would a Roman mathematician answer the equation X minus X or V minus V? What about V minus X?