The New Millennium
The question recently came up about the start of the new millennium. I wasn't going to get into it, but someone asked. In short, the official answer is, it starts in 2001. However, the most accurate answer is, we already missed it. (See what happens when you're not paying attention.) For more information, and a rather interesting story in a geeky sort of way, read on...
The year 2001 AD is the first year of the next millennium. The third millennium officially begins January 1, 2001 AD.
The reason, in short, is because the calendar we use today (the Gregorian calendar) was developed by Romans. The same Romans who used the Roman numeral system. The only real problem with the Roman numeral system is that they have no representation for zero. There is no Roman numeral for the number zero. In fact, as strange as it might seem today, the Romans did not understand the concept of zero. The Greeks did, but that knowledge was lost to the Romans. It was "rediscovered" when the Holy Roman Empire drove the Arabs (Moors) from Spain during the Crusades. The Arabs had somehow managed to retain the mathematical knowledge of the Greeks. Knowledge that included the concept of the number zero. (It took the Romans a while to wrap their minds around that one.) That is why to this day we use the Arabic numbering system (0,1,2,3, etc.), and not Roman numerals (I, II, III, IV, etc.).
The point of all this is that, since the Romans didn't have the number zero, they started their new calendar, based on the birth of Jesus Christ (A.D. - anno domini - year of our lord) with the year one (1 AD). There was no year zero. The end of the "first" year was Dec. 31st, 1 AD, and the "second" year began Jan. 1, 2 AD. Hence, the end of the first century ended with the passing of 100 hundred years, on Dec. 31st 100 AD, and the second century began Jan. 1st, 101 AD. Similarly, the first millennium ended at the end of 1000 AD, and the second millennium began Jan 1st, 1001 AD. So the next millennium (the third millennium since the birth of Christ) will really begin Jan. 1st, 2001 AD.
In any case, it doesn't really matter, since the Romans goofed up on calculating the exact date of the birth of Christ. Well, one particular Roman, the one who actually came up with the date. He was a monk named Dionysius Exiguus (aka. Dennis the Short). Best guess is that Christ was actually born sometime in 4 BC (and not on Dec. 25th either, but that's another story entirely). In reality, the millennium actually passed four years ago. (And nobody even noticed.)
Interestingly enough. Back in 999 AD, many people thought the world would end when 1000 AD rolled over. Not from the technical breakdown we fear now, but more of a real honest to God apocalypse of biblical proportion.
For more information on all this millennium stuff, refer to the following items from a variety of sources, including the US Naval Observatory; NASA; and the Royal Observatory Greenwich (keepers of Greenwich Mean Time [GMT], the time standard that the entire world uses to set their clocks); and others: