Taking for granted the accuracy of this traditional timekeeping system (or without regard to its significance), most folks will celebrate the departure of the old and the arrival of the new millennium in the year(s) 2000 and/or 2001.
But in fact the exact count of the years that have elapsed since Christ was born has over long centuries been lost track of, neglected, forgotten and mistaken so many times that the year numbers we routinely use today are nothing but crude approximations of the age of Christ.
The most credible testimonies and the most discernibly real traces that survive from his lifetime clearly indicate Christ was born several years prior to A.D. 1 (the supposed beginning of the Christian era). Modern scholars and first-century documentation place his birth possibly as early as 8 B.C. -- and most probably not later than late March in the year 4 B.C.*
Accordingly, whether he was actually born so late as 4 B.C. or, as seems more likely, even earlier, THE 2000TH ANNIVERSARY OF CHRIST'S HISTORICAL BIRTH (and hence the true end of the old millennium and the true arrival of the new one) CAN'T MATHEMATICALLY** NOR AT ALL REASONABLY BE POSTPONED VERY MUCH LONGER, INDEED NOT BEYOND ABOUT 6 OR 8 MORE WEEKS FROM THE PRESENT!
We may very plausibly (and perhaps must) continue to await the arrival of the actual, historical millennium until this coming March 13. [See notes, below.] Moreover, since there is a slight uncertainty about the exact dating, we may prefer to allow an extra margin of grace beyond this date and to settle, for symbolic reasons as well, upon the Vernal Equinox (March 20). Or, for more grace and symbolism, upon the full moon of Palm Sunday (March 23). Or for still more grace and symbolism, Easter Sunday (March 30). And at the very outer limit (yet still reasonably, aptly and commendably in view of all the data and circumstances), we could wait until the start of the new calendar month and trimester, for a clean slate and a real prodigy, that is, until ALL FOOLS' DAY, APRIL 1!
Past that point there is no reason (save a busted calendar) to postpone the inaugural celebration. The most perceptible, most definitive and truest version of the new millennium will already have arrived -- astonishingly early by any other reckoning, but beyond a shadow of a groundhog's doubt.
Hope to see you there!
Notes:
* Sources: The Gospels make it clear that Christ was born during the reign of Herod the Great, and the earlier, almost-contemporary chronicler Flavius Josephus states that Herod died "shortly after" the lunar eclipse that occurred on March 12-13 in [the Roman year equivalent to] 4 B.C. For a fuller and much more leisurely explanation and many more historical details please refer to "Jubilee" or Exactly When Does the New Millennium Actually Arrive Anyway? by Mr. Donner.
** As there was no year "0" between 1 B.C. and A.D. 1, one unit must be deducted from the otherwise simple addition of the A.D. and B.C. years, as follows: 1997 [years A.D.] + 4 [years B.C.] - 1 [nonyear "0"] = 2000 [years' passage]