When does a new decade begin, anyway?

We ushered in the '90s a year ago, but the last decade of the 20th century is only now about to unfold. Confused?Well, you can thank a monk named Dionysius Exiguus. He started it all many hundreds of years ago.

The question surfaces every 10 years, and for many people the answer is obvious: The new decade began with the 1990s a year ago on Jan. 1, 1990, and will end Dec. 31, 1999.

But don't tell that to the calendar purists.

"If you refer to the decade of the '90s, I'm willing to concede that 1990 is in the '90s. It's obviously not in the '80s," says LeRoy Doggett, a calendar expert at the U.S. Naval Observatory.

But he explains that this view of a decade in time is simply a colloquialism with no basis in history.

"When we talk about the last decade of the 20th century, we get into a chronological question that forces you to ask: When did the first century begin?" he says.

Doggett provides this explanation.

The numbering system in our current Gregorian calendar, indeed in all human calendars, is arbitrary. Someone picked a particular date or event and started counting from that point. There is nothing in nature that causes it to be a particular year, or for a particular day to be New Year.

That's where the monk Dionysius Exiguus, or "Dennis the Small," got into the act nearly 1,500 years ago. He had been given the task of developing a set of tables to help local priests and bishops determine the date of Easter.

At the time, everything was dated from the accession of the Roman emperor Diocletian. But Diocletian had persecuted the early Christians, and Dionysius didn't want to commemorate him in the new tables.

Instead Dionysius based the tables on the date of Jesus' birth, which the monk determined had occurred 532 years earlier. Dionysius called the year of Jesus' birth the year "1" and began the count.

"So the first decade was 1 to 10, the second was 11 to 20. Extrapolating from that, we end up starting a new decade with 1991, which runs through 2000, completing the 20th century. So the 21st century starts with 2001," said Doggett.