There is no such thing as a year 0; at least not in the Gregorian calendar. The year AD 1 is the first year succeeding the hypothetical birth of Jesus. AD 1 follows thus directly after 1 BC. Voila! No year 0!
All this makes for some peculiar mathematics. For example between the year 500 BC and the year AD 500 there are not 1000 years; there are 999 years! 500 of those taking place BC, the other 499 AD. This is how historians number the years. Astronomers are of course in need of mathematical accuracy, and have therefore added a leap year zero – equal to 1 BC. ISO is for obvious reasons also in need of accuracy, and has added a zero, which in turn is written as 0000, equalling the historical year 1 BC. This will, as far as I can understand, funnily enough make historical events designated to a certain year, different in astronomical or ISO count, compared to the commonly used Gregorian count. For example would the first Roman invasion of Britannia have happened in year 55 BC according to the historians and the Gregorian calendar, but in 54 BC according to the astronomers. Would it be –0054 according to ISO …? Well, yes I suppose so.
Another thing about the count starting on one, instead of on zero, is that this would make the first decade of a century nine years, and the last decade eleven years. Of course they aren’t even fit to be called decades in such case.
Now, to the really interesting bit! As far as I can understand we must have celebrated the turn of the century a year to early! Surely two centuries would not have elapsed until AD 2001 if the count started at AD 1 … Shame I didn’t come to realise this some five and a half years ago, as I then could have been in for another big millennium celebration!
And now to the calendar of political correctness: The Holocene calendar. This is a suggestion for a new calendar, on the premises of the Gregorian calendar being offensive to non-religious or non-Christian people. The counting of the current era, according to this calendar, would start with the approximate beginning of the current geological era, 10 000 years BC. 1 BC will be year 10 000 HE, AD 2006 will be 12006 HE. No big deal: just put a 1 in front! Apart from us getting away from the religious aspect of the Gregorian calendar, using the Holocene calendar, we would also make rid of the problem of the missing year 0. This calendar would not be very popular with the US Evangelistical neocons though, I am sure ...
Some curiosa: The difference between the Gregorian calendar and for example some Hindu or Buddhist calendars, is that those will count elapsed years, such as one does when counting a persons age. Thus, they will start the count at zero, and therefore of course have a year zero.
I love you Wikipedia!