o, you idiot. No, you idiot. No, you idiot. No, you idiot. No, you idiot. No, you idiot. No, you idiot. No, you idiot. No, you idiot. No, you idiot. No, you idiot. No, you idiot. No, you idiot. No, you idiot. No, you idiot. No, you idiot. No, you idiot. No, you idiot. No, you idiot.

THE ROYAL OBSERVATORY GREENWICH SAYS:
"When do the 3rd Millennium and the 21st Century start?
"A millennium is an interval of 1000 years and a century is an interval of 100 years. In the Gregorian Calendar, which we use, there is no year zero and the sequence of years near the start runs as follows; ..., 3BC, 2BC, 1BC, 1AD, 2AD, ...

"Because there is no year zero, the first year of the calendar ends at the end of the year named 1AD. By a similar argument 100 years will only have elapsed at the end of the year 100AD. Since 2000AD is the 2,000th year of the Christian calendar, it will be the last year of the Second Millennium. So the 3rd Millennium and the 21st Century will begin at the same moment, namely zero hours UTC (commonly known as GMT) on January 1st 2001.

"We have received a great deal of e-mail regarding the start of the 21st Century. It is interesting to note that this is not the first time that this controversy has arisen.
"The Times [of London] must have received many letters towards the end of 1799, since its editors felt moved to make the following comments about the beginning of the 19thCentury:

"'We have uniformly rejected all letters and declined all discussion upon the question of when the present century ends, as it is one of the most absurd that can engage the public attention, and we are astonished to find it has been the subject of so much dispute, since it appears plain. The present century will not terminate till January 1, 1801, unless it can be made out that 99 are 100... It is a silly, childish discussion, and only exposes the want of brains of those who maintain a contrary opinion to that we have stated' The Times, 26 December 1799"
--From Special Information Leaflet No. 29: `The New Millennium' By the Royal Observatory Greenwich

Now, they're the people who set what time it is all over the world, so they ought to know.

And the U.S. Naval Observatory says:

Years of the Gregorian calendar, which is currently in use today,
are counted from AD 1. Thus, the 1st century comprised the years
AD 1
through AD 100. The second century began with AD 101 and
continued through AD 200. By extrapolation we find that the 20th
century
comprises the years AD 1901-2000. Therefore, the 21st century
will begin with 1 January 2001 and continue through 31
December 2100.

Similarly, the 1st millennium comprised the years AD 1-1000. The
2nd millennium comprises the years AD 1001-2000. The 3rd
millennium
will begin with AD 2001 and continue through AD 3000.