As the year AD 2000 approached, stayed briefly, and then receded into history, there was a good deal of self-righteous argument about "when the Third Millennium/Twenty-First Century began."
Said one group: The Third Millennium/Twenty-First Century AD does not begin until the year AD 2001, because as everyone knows, there was no year zero.
Said another group: The calendar is an arbitrary creation of humankind, and we can interpret it however we please. And we please to have the Third Millennium/Twenty-First Century AD begin with the nice, round year of AD 2000, just like a car odometer. So there, phhhtt!
So's Your Mother, replied the AD 2001 people. You're wrong, wrong, wrong, and we forbid you to celebrate on December 31, 1999.
If we're wrong, it's only by one part in a thousand, said the Odometer crowd.
Wrong is wrong, countered the AD 2001 folks. Take off those silly hats and put down that champagne!
Go ahead and sulk, replied the Odometer crowd. We're going to party!
The AD 2001 contingent promptly retreated to a corner and whined, You're ruining this Millennium for everyone.
In fact, when Dionysius Exiguus created the whole Anno Domini chronology c. AD 525, he calculated that the birth of Christ had occurred in 753 AUC, and used January 1, 754 AUC as Day One of his system. And he was wrong. According to what we now know of history, and maintaining consistency with the Gospels, Christ could not have been born any later than 4 BC.
The Third Millennium after the Birth of Christ, then, happened in AD 1996 (or 1997, depending upon your reasoning), and by the time the AD 2001 people started their bitching, they had already missed it.
Besides, according to Dionysius Exiguus' own calculations, the Third Millennium AD actually started on December 25, 2000.
Meanwhile, the Odometer Group managed to celebrate twice -- on December 31, 1999, when they believed that the new Millennium began, and also on December 31, 2000, when the Odometer crowd insisted that everyone celebrate.
In hindsight, humanity seems to have come to the unspoken conclusion that the turn of a Century (or especially a Millennium) is too big an event to happen all in one night. Instead, we spent the whole year AD 2000 getting used to the idea, phasing the new Millennium in as it were. In a global compromise, we all decided that AD 2000 was the last year of the 20th Century/Second Millennium, and was simultaneously the first year of the 21st Century/Third Millennium.
(In the Holocene system, there is no issue. At Meerkat Meade we celebrated the beginning of the 12,000s at midnight on December 31, 11.999.)
One of the most abysmally stupid comments I ever heard was that of a person who wrote into a magazine, indignant over a perceived error. "You said that the presidential election of 1960 took place in the decade of the Sixties, while everyone knows that the Sixties didn't start until January 1, 1961."
Sigh.
This idiot's position might have been defensible, if the magazine had referred to the election as taking place in "the seventh decade of the Twentieth Century." If you go along with the idea that the 20th Century started on January 1, 1901, then the seventh decade of that century didn't start until January 1, 1961.
But the "decade of the Sixties" refers to any year of the form "nineteen sixty-___"
Which, of course, raises the question of what to call the decade from AD 2000 to AD 2009. The answer will no doubt evolve, probably around the middle of the decade, but our preference here at Meerkat Meade is to take a leaf from James Bond and call this decade the "double-ohs" For example, "Way back in double-oh six...."
But we're not going to worry about it right now. We're still busy waiting for people to stop saying "two thousand and one" (or, for the more literate, "two thousand one") and join us in saying "twenty-oh-one." After all, we don't say "nineteen hundred one," we say "nineteen-oh-one." We hate to think of getting halfway through the century and still saying "two thousand forty-nine."