ARITHMETIC and common sense. We so often use the former to bolster the latter - producing hard numbers to deflate squishy fantasies, or declaring something ``sensible'' because ``it all adds up'' - that we almost think of them as synonyms. On those rare occasions where they contradict each other, we're profoundly baffled. So brace yourself for the ultimate bafflement. Common sense says that the 21st century begins at midnight on Dec. 31, 1999. Just as your car has gone 50,000 miles when the odometer rolls up a five and four zeros, so the calendar, when the great cosmic odometer rolls up 2000, will have entered the new millennium. What could be more obvious, right?
Wrong. In rushes arithmetic to prove that the 20th century, like an outvoted dictator refusing to step down, will linger on until Dec. 31, 2000. The year 2001 actually inaugurates the new century.
Why so? Because, pundits explain, there was never a year 0. The year 1 BC gave way immediately to AD 1. Well, not exactly, they confess, because at the time no one knew it was AD 1 anyway. How could they, since what happened in Bethlehem that year wasn't yet recognized as the beginning of the Christian era? It was only in retrospect that the year 1 got itself appointed - and by then nobody wanted to describe the first year of Christianity as a zero.
And there's the rub. By the beginning of the year 2, only one year had elapsed. By the year 10, only nine had passed - and, by 1900, only 1,899. No question about it: Before the 21st century begins, 20 centuries must have ticked away. But only 19.99 will have gone by when 2000 begins.
What throws us off is the metaphor of the odometer. Cars, after all, start at zero: When it says ``000001.0,'' you've already gone one mile. How to preserve sanity? Scrap the metaphor - which, of course, is easier said than done. The illusion of common sense is a powerful thing - so potent, in fact, that I'm guessing that the brass bands and brouhaha attending New Year's Eve, 1999, will be far more rambunctious than the celebrations a year later. The result: Yet another battle between the purists and the hoi polloi - as wearying and interminable as those still being fought over split infinitives, ``different from'' instead of ``different than,'' and the proper spelling of donut. The upshot, of course, will be that the greatest global celebratory moment in 1,000 years will be dragged out for a full year, with neither midnight getting the full authority it deserves.
So I have a proposal. Let us simply declare, for once and for all, that new centuries begin in the even-numbered years. Let's swallow hard, define the 20th century as a truncated period of only 99 years, shift ourselves to the proper track, align mathematics and common sense, and save future generations this maddening centennial debate.
``And tinker with the calendar?'' I hear the purists blustering. ``Not on your life!''
Nice argument, but flawed. History is full of tinkering. Look at 1582, when things got so out of kilter that 10 days had to be omitted from the year. The problem: a year is not 365.25 years, but 365.242199. Try as you will, you can't keep things on track even with leap years. Happened again in 1752 - the year that, by edict, Sept. 2 was immediately followed by Sept. 14.
Compared with that kind of tinkering, my proposal is painless. And given the number of books and articles already appearing with the year 2000 in their title, it sure would save us a deal of grief.