The Gregorian Calendar which is followed in most western and western-oriented countries now has been "adjusted" several times in the few hundred years of use. Even now, it has to be "adjusted" for astronomical actualities every four years (leap year). And yet, it is still off a fraction of astronomical time which needs to be further corrected ever now and then. In astronmical terms, it is a rusty and dull instrument.
The "timing" of the birth of Jesus, The Christ, is wildly inaccurate, off by as much as 20 years according to some researchers. Most who study such things tend to agree that 1 A.D. Gregorian was actually somewhere around 3-6 B.C. A.D. and B.C. have been replaced by C.E. [Common Era] and Before Common Era [B.C.E.] in deference to the majority of world peoples using the Gregorian calendar who are not Christians by choice.
The decimal system, counting by tens, by which an alleged Millennium is measured as 1,000 years (earth/sun cycles) is a human fabrication having nothing to do with astronomy and most to do with the number of digits on our two hands or two feet. We could count just as well in any number of other systems whether binary (which runs the software guiding this machine) or any other base of number/symbol adopted as a convention.
Whether the "mil-looney-um" is pure chauvinism, xenophobia or simple ignorance is debatable, even if a year early by the Gregorian decimal system calendar in use.
The "Lonely Planets" newsletter provides some perspectives [edited and amended a bit here and there]:
"Although this new year is being hyped like no other, those who use calendar traditions other than the Gregorian calendar--at least half the world's population--may be wondering what all this millennium fuss is all about.
"The Chinese, Koreans and Vietnamese, for instance, follow a lunar calendar, so their new year falls on a different date from year to year. Next year, the Year of the Metal Dragon [a twelve year cycle or sequencing of years] begins on 5 February by the Gregorian Calendar.
While Japan officially now celebrates new year on 1 January, its celebrations last a full week. Traditionally, Japanese have named and counted their years by the length of an emperor's reign. The count starts all over again with each new Emperor. Next year, 2000 by Gregorian counting, will be 12 Heisei (the present Emperor) in the traditional system.
Although the Indian subcontinent uses many calendar systems, officially, the Hindu Saka year of 1922 begins on 21 March, 2000 C.E.
Kashmiris mark new year during Nau Roz, a major spring festival through the central Asian region. Baisakhi is the new year in Punjab as well as the day Sikhs celebrate the founding of their faith. That party starts 13 April. 14 April is new year for Bengalis -- Naba Barsha in Bangla, the province formerly known as Bengal and Pohela Baisakh in Bangladesh. It is also Nepali new year -- in fact, the Nepalis are entering year number 2057 by their traditional calendar -- as well in Kerala in Southern India, when locals celebrate Vishnu, the first day of the Malaysian calendar.
The following day, 15 April, is Tamil New Year. In the west Indian province of Maharastra, with Mumbai (formerly Bombay) as its capital, the festival of Gudi Pavda usually kicks off in March or April at the beginning of the Hindu month of Chaitra.
Before the arrival of the Spanish Conquistadors, the Mayans were famously obsessed with time, so much so in fact that they devised and used two calendars. The first one was used for sacred purposes and operated on a 260-day cycle, while the other was secular and based on a cycle of 365 days: the Mayan new year falls on 26 July.
Every major religion seems to have developed its own methods of counting the passage of years. On 6 February, 2000 Gregorian, Tibetans will celebrate Losar 2127 as will Bhutanese and others of the region.
Muslims celebrate Hijra year 1621 on 6 April. Parsi new year comes on 22 August [Parsi, Persian-based system based deep in history - Zoroastraian, etc.] to mark the escape of their ancestors from Persian persecution.
Jews will welcome Rosh Hashana 5761 next year on 30 September. India's Jains will celebrate new year 2526 on 25 October which also occurs during the Hindu festival of Divali for their year 1120.
And so it goes throughout the planet that others are puzzled, yet again, by the strange antics and obsessions of folks like us.