A millennium (plural millennia or millenniums) is a period equal to 1000 years, also called kiloyears. It derives from the Latin mille, thousand, and annus, year. It is often, but not always, related to a particular dating system.
Sometimes, it is used specifically for periods of a thousand years that begin at the starting point (initial reference point) of the calendar in consideration (typically the year "1"), or in later years that are whole number multiples of a thousand years after it. The term can also refer to an interval of time beginning on any date. Frequently in the latter case (and sometimes also in the former) it may have religious or theological implications (see millenarianism). Sometimes in use, such an interval called a "millennium" might be interpreted less precisely, i.e., not always being exactly 1000 years long. It could be, for example, 1050, etc.
There are two methods of counting years: current years (the count begins at the epoch) and elapsed years (the count is of completed years since the epoch). This latter method is used in India.
The original method of counting years was ordinal, whether 1st year AD or regnal 10th year of King Henry VIII. This ordinal numbering is still in the names of the millennia and centuries, for example 1st millennium or the 10th century, and sometimes in the names of decades, e.g., 1st decade of the 11th century.
The main issues arise from the content of the various year ranges. Similar issues affect the contents of centuries. Decades are usually referred to by their leading numbers and are therefore immune to this controversy: the decade called 1990s would by its naming not include 2000. Similarly the century called the 1900s includes 1900 but not 2000.
Those following ordinal year names naturally choose
Those who are influenced by the leading digit equally naturally choose
The common Western calendar (the Gregorian calendar) has been defined with counting origin 1. Thus each period of 1,000 years concludes with a year number with 3 zeroes, e.g., the first thousand years in the Western calendar included the year 1000. However, there are two viewpoints about how millennia should be thought of in practice. One viewpoint relies on the formal operation of the calendar, while the other appeals to other notions that attract popular sentiment. Stephen Jay Gould argued that the choice is arbitrary, and since the question revolves around rules made by people, rather than a natural phenomenon that is subject to experimental measurement, the matter cannot be resolved.
The ISO 8601, employed in a number of contexts, uses the astronomical calendar, in which year counting starts at 0. Thus, when using this calendar, the millennium starts at x000 and ends at x999. There was a popular debate leading up to the celebrations of the year 2000 as to whether the beginning of that year should be understood (and celebrated) as the beginning of a new millennium. Historically, there has been debate around the turn of previous decades, centuries, and millennia.
The issue is tied to the convention of using ordinal numbers to count millennia (as in "the third millennium"), as opposed to "the two thousands", which is unambiguous as it does not depend on which year counting starts. The first convention is common in English speaking countries, but the latter is favored in for example Sweden (tvåtusentalet, which translates literally as the two thousands period).
Those holding that the arrival of the new millennium should be celebrated in the transition from 2000 to 2001 (i.e., December 31, 2000 to January 1, 2001), argued that because the Gregorian calendar has no year zero, the millennia should be counted from 1 AD. Thus the first period of one thousand complete years runs from the beginning of 1 AD to the end of 1000 AD, and the beginning of the second millennium took place at the beginning of 1001. They also point out that it's mathematically impossible for the third millennium to have begun when 1999 ended regardless of who was in favor.
|2 BC||1 BC||1 AD||2||3||4||5||...||998||999||1000||1001||1002||1003||...||1998||1999||2000||2001||2002||2003||...||2998||2999||3000||3001||3002||3003||...|
|First one thousand years (millennium)||Second millennium||Third millennium||Fourth millennium|
Arthur C. Clarke gave this analogy (from a statement received by Reuters): "If the scale on your grocer's weighing machine began at 1 instead of 0, would you be happy when he claimed he'd sold you 10 kg of tea?" This statement illustrates the common confusion about the calendar.
If one counts from the beginning of AD 1 to the ending of AD 1000, one would have counted 1000 years. The next 1000 years (millennium) would begin on the first day of 1001. So the calendar has not 'cheated' anyone out of a year. Clarke made reference to this viewpoint in his book 3001: The Final Odyssey referring to the Millennium Celebrations on December 31, 2000. In other words, the argument is based on the fact that the last year of the first two thousand years in the Gregorian calendar was 2000, not 1999.
The "year 2000" has also been a popular phrase referring to an often utopian future, or a year when stories in such a future were set, adding to its cultural significance. There was also media and public interest in the Y2K bug. People liked to compared their odometer as a reason to celebrate the new millennium which goes from 1999 to 2000. Thus, the populist argument was that the new millennium should begin when the zeroes "rolled over" to 2000, i.e., the day after December 31, 1999. People felt that the change of the hundreds digit in the year number, and the zeros rolling over, created a sense that a new century had begun.
This is similar to the common demarcation of decades by their most significant digits, e.g., naming the period 1980 to 1989 as the 1980s or "the eighties". Similarly, it would be valid to celebrate the year 2000 as a cultural event in its own right, and name the period 2000 to 2999 as "the 2000s". In other words, the time period between 1 and 999 (999 years only) would be called the "0s" and the period between 1000 and 1999 would be the "1000s".
|First millennium (1000 years)||Second millennium||Third millennium|
|1 BC||1 AD||2||3||4||5||...||998||999||1000||1001||1002||...||1998||1999||2000||2001||2002||...||2998||2999||3000||3001||3002|
|First millennium (999 years only)||Second millennium||Third millennium|
The majority popular approach was to treat the end of 1999 as the end of a millennium and to hold millennium celebrations at midnight between December 31, 1999 and January 1, 2000, as per viewpoint 2. The cultural and psychological significance of the events listed above combined to cause celebrations to be observed one year earlier than the formal Gregorian date. This does not, of course, establish that insistence on the formal Gregorian date is "incorrect", though some view it as pedantic.
Some event organisers hedged their bets by calling their 1999 celebrations things like "Click" referring to the odometer-like rolling over of the nines to zeros. A second approach was to adopt two different views on the millennium problem and celebrate the new millennium twice.