At midnight Thursday, the world enters the last year of this century, the last year before the beginning of a new millennium -- right?

NO! insist the persnickety official timekeepers of the world.YES! respond millions who are already booking flights to resorts where they plan to celebrate the arrival of the third millennium on Jan. 1, 2000.

"The New Zealand Millennium," sponsored by the government of that country, is planning celebrations for Jan. 1, 2000, featuring the "millennium bell," called the world's largest tuned bell; an international children's festival; "Eco 2000, a conservation project of world significance" and many other events.

New Zealand is especially giddy about the turning of the millennium because, in the worlds of the Southern Millennium Club, "The towns of southern New Zealand are located just minutes away from the international date line and are among the first to see the dawn of each new day." In other words, the calendar rolls over first at the date line, and New Zealand is the first major country after it does.

Many who want to be among the first to see in the new age plan to fly to New Zealand, to the delight of the New Zealand tourist industry.

However, Greenwich, England, is putting up a challenge for millennial bragging rights.

The Greenwich 2000 Network argues that the millennium actually starts there. Backers of this theory point out that during an international conference held in Washington, D.C., in 1884, leaders agreed to have the "universal day" begin at the Greenwich Meridian, which runs through the Royal Greenwich Observatory.

This prime meridian is where the day officially starts, say the champions of Greenwich, where a huge millennium dome has been built over the imaginary line. "Hence each and every day begins in Greenwich, including of course 1 January 2000," says an Internet site established by the network.

The international date line is where a new date begins. It runs along the 180th Meridian from north pole to south pole, going through the Pacific Ocean. The shakiness of England's claim is proven by the fact that when Jan. 1 begins in New Zealand, the time will be noon Dec. 31 of the previous year, in England.

After making its claim for priority, the Greenwich 2000 Network quickly adds, "although the millennium officially starts on 1 January 2001."

Says who?

Says the U.S. Naval Observatory, for one. The observatory, located in Washington, D.C., is the country's official timekeeper. Because of its great precision, it's the unofficial timekeeper for the whole world.

The observatory has been ringing out the time since 1845. Today it uses sophisticated cesium and hydrogen maser clocks to keep track of time with fearful accuracy: continuously updated, its rate does not vary by more than 100 trillionth of a second from day to day.

As precise as its time-telling is, the observatory shows evidence of hesitation about when the millennium starts.

"Countdown to the Millennium" proclaims an Internet site maintained by the observatory. It tells how long it is until the year 2000, in days, hours, minutes and seconds.

Then the same page offers the sizable caveat, "Actually, this clock is counting down to the year 2000 . . . The next millennium technically doesn't begin until Jan. 1, 2001."

According to the Naval Observatory, the calendar system we use, the Christian calendar, begins counting with A.D. 1. "Thus, the first century comprised the years A.D. 1 through A.D. 100," it says. "The second century began with A.D. 101 and continued through A.D. 200.

"By extrapolation we find that the 20th century comprises the years A.D. 1901-2000. Therefore, the 21st century will begin with 1 January 2001 and continue through 31 December 2100.

"Similarly, the first millennium comprised the years A.D. 1-1000. The second millennium comprises the years A.D. 1001-2000. The third millennium will begin with A.D. 2001 and continue through A.D. 3000."

How could something so counterintuitive become enmeshed in the fabric of our lives, in the very way we measure our years?

"Every society has its own calendar system, and that calendar system is based on some important event in the past," said Paul Pixton, history professor at Brigham Young University, Provo. In the Bible, events usually are dated from the start of somebody's reign, he said.

"That's the way the ancients did it," whether they were Babylonians or Hebrews.

The Chinese have a calendar system that dates to about 2000 B.C., the Muslims start Year 1 with the flight of Mohammed from Mecca in what the Western calendar calls 632.

"That's one of the reasons why dating in the ancient world is a very difficult thing," Pixton said. Each group had a different system.

"There are about four different dating systems that are referred to in the Book of Mormon, and it's very consistent with the way ancient people dated things."

The starting point for the calendar of the Roman empire was the legendary founding of Rome with the arrival of the infants Romulus and Remus. Ancient Romans dated this to a year that corresponds to 753 B.C. in our calendar.

Romans in the classical period would refer to a year as "ab urbe condita" (from the founding of the city) and then a number to tell how many years.

This was not a satisfactory system to Christians after the fall of Rome.

In the year 532, an English monk named Dionysius Exiguus crafted the Christian calendar and was able to find a correspondence with a Roman calendar. In his great work, Jesus Christ's birth was placed at Anno Domini 1 -- which translates as, "in the year of our Lord 1."

Modern scholars think Dionysius Exiguus was off a few years, that Jesus probably was born in 4 or 5 B.C. But the point has little to do with argument over 2000 vs. 2001.

In the eighth century of the Christian era, an English historian called the Venerable Bede extended the calendar by inventing the system we use to refer to dates prior to 1 A.D. He came up with the B.C., or Before Christ, system of designating events before Jesus.

However, Western counting systems did not use zero in those times. There is no zero year in either the Dionysius Exiguus or the Bede systems. Years are numbered: 3 B.C., 2 B.C., 1 B.C., A.D. 1, A.D. 2, A.D. 3 . . .

That means the first year of Jesus' time is Year 1, not Year 0. The decades, centuries, millennia all start with 1 and end with 0, according to the purists.

When to celebrate the turn of the millennium? Despite the quirks built into the calendar by its founders, most people recognize Jan. 1, 2000, as the turning of the new leaf.

"Everybody is thinking in terms of the new millennium starting in 2000," Pixton said. He thinks it will be "virtually impossible" to change that impression. "I guess the purists could hold out for 2001," he added.

As the New Zealand government puts it, technically 2001 is the beginning of the new millennium. "But for most of us, the main event starts when the calendar starts reading 2000."