• well . IF the year 0 is both AD and BC than I'm gonna say 21, but there is no year zero .. do you count to ten starting with zero or one ?
• 20 don't make it hard on yourself.
• There is no such a thing as Year 0, and this is why. Please understand that we do not count years as "one, two, three, etc.". (The bogus idea of the "year zero" comes from this misconception.) We count years as "first, second, third, etc." There is no natural need in something like "zero'th". Even though they sometimes use it in math like linear algebra or in computer science out of convenience, this is an artificial construct, not needed in everyday life of ordinary people. The idea of "year 0" comes from peculiarity of written English, where we write "Year 2000" instead of "the 2000th year". It does not exist in other languages. The true meaning of "1 AD" is "the first year of Our Lord", not "year one of Our Lord". Some people believe (I am not discussing religious issues and beliefs here) that a certain Jesus was born on December 25 (which is "the twenty fifth day of December", not "the day 25 of December"). Neglecting a few remaining days, the next year became "the first year of the era of Our Lord" or 1 AD (or, as those who are not Christians but are forced to use the same approach to counting years because of historical circumstances and tradition write, 1 CE - "the first year of Common Era"). By the same token of neglecting a few days, the year in which Jesus was born became "the first year before Christ", i.e. 1 BC. Just remember, it is "the first year", not "year one". Therefore, there is no place for "year zero", this is just nonsense. Mathematically, the whole thing is nothing but confusion between ordinal and cardinal aspects of the concept of a number. "Ordinal" describes and expresses order of things, while "cardinal" - quantity. They are related but not the same. Historically, those two aspects where independent of each other. You can see that from the names of the several first (or small!) numbers used strictly in accordance with their meaning: first - one, second - two. The names have nothing in common, which indicates independence of their origins. "Three" and "third" resemble each other, but only beginning with "four" and "fourth" you can see a clear relationship. In languages other than English the situation may be even more pronounced. So, forget about "Year 0". Depending on whether you want to include or not include the years defining the time interval you asked about, there are 20, 19, or 18 years in it.
• No Zero Year. Ancient peoples, including the learned Greeks, the Romans, and the Jews, had no concept of zero. To them, everything began counting from one. When you studied Roman numerals in school (I, II, III, IV, V, X, etc.), did you learn a figure for zero? No, because the Romans had none. Since the Romans did not use the number zero, the Common Era began, not with a zero year, but with 1 C.E. This also gave rise to the ordinal arrangement of numbers, such as first (1st), second (2nd), third (3rd), tenth (10th), and hundredth (100th). In modern mathematics, man conceives of everything as starting from nothing, or zero. The zero was probably invented by the Hindus.