When you celebrate the arrival of a new year tonight, whoop it up. For you also celebrate the beginning of a new decade. Never mind all the misguided folderol that was expended a year ago. That was based on an incorrigibly faulty understanding of how the calendar works.

The year that ends today is the last year of the ninth decade of the 20th century. It takes 90 years to make nine decades, and the year that starts Tuesday is the first year of the 10th and last decade of the century. Since it will take 10 more years to make one more decade, the year 2000 will be the last year of the 20th century, not the first of the new century.

Sure, if you want to call this the decade of the '90s, 1990 was the first of those years. But that approach raises a problem that will be evident in nine years. What will we call the decade after the Nineties? The Decade of the Oughts? And the decade after that?

Better, calendar purists say, to recognize that our method of numbering years is based on the assumption that the first year of the numbering system was the year 1. Thus the first century A.D. ended with the completion of 100 years. That was, obviously, the year 100. The first year of the second century was the year 101. And so on.