Heh.

• nope, there was never a year "zero". When Denis the Short first came up with the current BC/AD year thing, the Romans at the time had no real concept of a mathematical zero (that wouldn't show up at all until the 11th century in Damascus, and not in the Roman/European empires until the 13th century or so, when nobles and traveling scholars had brought it back home from the Crusades). So, no "Year 0". Note that there is a difference between the number zero and a mathematical one... "0" (or "null") was what had been missing all that time, not the digit itself (which sits just fine at the tail end of "10" and such, though the Romans always had their particular numbering system - we're using the Arabic-built version these days, though few used it to mark years until roughly the 16th or 17th centuries - forget which).

* The funny part is, the true counting of AD ("Anno Domini", or "In the year of Our Lord") was goofed anyway, since Denis forgot that Augustus Caesar (long name: Imperator Caesar Divi Filius Augustus) had ruled under a different name before becoming the emperor named at the time of Christ's birth, which sort of threw off the calculations by four years (Denis was counting up roman years by counting 'nth years of the reign of {insert emperor here}'. )

• Because of that last two points, IIRC this would properly be called 2005 AD (or so) if we were to go back and correct all the big messes. Even then, calendars weren't all that accurate until the 16th century, so give or take a week or so if you want anything more accurate than what year we're in. Even then, thanks to the whole Julian vs. Gregorian calendar thing, odds are good that we may even still have to give or take a year.

* Because everything is noted and timed (calendar-wise) fairly accurately for about 500+ years now, if we did correct for the goofs, historians would find it too messy to keep up, so don't expect any fixes any time soon.

• The Hebrew calendar says we're in the year 5770 - you'll have to ask Mr. Hoadley where they based their first year. I think it's a pure Lunar cycle, but I'm not 100% sure.

* The Islamic calendar says we're in the year 1431 - the number of years since Mohamet moved from Medina to Mecca for good - a move called the Hijra. Thing here is, the Islamic calendar runs off a strict Lunar cycle... so it's always a few days shorter than our current Gregorian calendar. 

* All existing (and used) calendars are based on some event, almost always religious. Even the atheist/agnostic BCE/CE is still based on a presumption of when the birth of Jesus Christ occurred. The standard global civil calendar is known as the Gregorian Calendar, which was established by a Pope.

• The 21st century began on January 1st, 2001. They had a fairly big fight about this back in 1900, too.