Disclaimer: I am an amillennialist in that I do not believe there to be any special significance to this "Third Millennium" business. I am not a Christian, so this "Second Coming" business also means nothing to me, except for concern over what it may drive believers to do. I refer to the Third Millennium here only because many others use the term and consider it significant.
2001 Jan 01 was the first day of the 21st Century and the Third Millennium.
That's right, the entire year of 2000 was still in the 20th Century. If you do not understand why that is, then I will explain.
Circa 525 CE (Common Era, the prefered non-sectarian nomenclature for "Anno Domini"), the monk, Dennis the Short (Dionysius Exiguous), created the Anno Domini reckoning of the years. He calculated back to determine when Jesus was born and started counting the years from that point on.
The problem is that Dennis didn't know about nothing. That is to say, he didn't know about zero. Nobody in Europe did, at least not until the Moors imported the pagan Hindu numbering system, which we still call "Arabic numerals", into Spain after 800 CE. Since Dennis the Short didn't know about zero, he designated the year of Jesus' birth as the year "one AD." There was no year "zero" in his system.
From here, it is a simple matter of the simplest form of mathematics: counting. Let's count 100 items. The way that people normally count things. The way that Dennis counted the years. You count the first item as "one", the second as "two", and so on until you count the 100th item as "100." Thus, the years 1 through 100 formed the first century CE, 1901 through 2000 formed the twentieth century, and 1001 through 2000 formed the second Millennium.
Thus the year, 2000 was the last year of the 20th Century and the 21st Century did not start until 2001 Jan 01. Therefore, we are still in the 20th Century when we celebrated New Year's Day 2000.
QED
"So what?", you say. "What does it really matter how the first year was numbered? Why do you have to be such an insufferable self-righteous math geek? Why do you want to try to spoil everybody else's fun? Why can't you simply ignore the facts like everybody else and mindlessly follow the herd? Just pretend that the first century was only 99 years long. Does it really matter?"
Well actually, no, it doesn't really matter. There is nothing magical about 2000 (except in computers), nor 2001, nor 1984, nor 1951, nor 1776. There is no magical or natural significance in when a decade, century, or millennium starts; I am an amillennialist, after all. Nor is there any magical or natural significance in when a year starts. Aside from the need for a solar calendar to remain in sync with the solstices and equinoxes, the entire matter of calendars is completely arbitrary. Aside from the need for a clock to remain in sync with the Sun crossing the meridian, the entire matter of hours, minutes, and seconds is completely arbitrary.
Except for the need to keep in sync with the natural cycles upon which they are based (note that the month no longer does this, except in lunar calendars), our systems for enumerating time are completely arbitrary and devoid of any inherent meaning. Therefore, it does not really matter what we designate as the start of a new calendar/clock cycle; it can carry no significance except for what we arbitrarily assign to it. Numbering the decades and the centuries and the millennia serves no purpose except to provide historians with a convenient way to refer to a historical period. Does it really matter whether we are in the 20th Century or the 21st or the 58th? No, not in the least.
And yet ...
Even though our calendars and clocks are arbitrary systems, they are also conventions upon which our society is built. Our ancestors agreed to those systems and have created authorities to administer and regulate them, along with several other conventions and standards. We are bound to abide by these conventions and standards and we depend on them. Sure, 2000 January 08 at 0730 PST is an entirely arbitrary designation for a particular moment in time, but we have all agreed on which particular moment in time that represents and if we schedule an event for that time on that date, then everybody will have agreed on when it is scheduled.
That schedule and every other schedule that we have are all based on the date/time conventions of our society. We cannot arbitrarily choose to ignore these conventions piecemeal. If we were each to start dreaming up our own personal versions of the calendar or of the time of day, then nobody could agree on when anything would happen and chaos would ensue. Only a duly appointed authority may make changes in the calendar or the time of day, such as the National Institute of Standards and Technology (NIST).
We have all agreed, implicitly through our ancestors, to accept and follow the date/time conventions of our society. We are not free to pick and choose which parts to follow and which not to; we are bound to follow them all. Our society uses Dennis the Short's Anno Domini numbering system, which starts with the year One and in which each century starts with year xx01 and ends with year xy00; e.g., the 20th Century started with the year 1901 and will end with the year 2000. That is the convention that we are supposed to follow. Refusal to follow society's conventions and standards can only cause confusion.
This matter and its history is discussed much more fully on "Blame the madness on Dennis the Short" at http://www.ianchadwick.com/essays/madness.html (or you can Google the title; the article is reprinted on several sites). This page also contains several links on the subject, including to the official time-keepers: the National Institute of Standards and Technology (NIST), the US Navy Observatory, and the Royal Observatory in Greeenwich (all of whom officially stated that the Third Millennium wouldn't start until 2001 January 01).
Uploaded on 2000 January 20.
Last updated on 2011 July 08.