University of Botswana History Department

When is the Millennium?

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This page has been added in response to a number of inquiries at UB about the date of the new Millennium. Is it 2000 or 2001? The question does in fact have a definite right answer.

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The date of the Millennium

Although many of us were celebrating New Year 2000 as the start of the new millennium, the Third Millennium does not, strictly speaking, start until 2001. Why?

The western calendar, with BC/AD (or BCE/CE) dates, was invented by the monk Dionysius Exiguus in the 6th century AD. At that time, the number Zero was not in use by European mathematicians, and so Dionysius did not include a Year Zero. The sequence of years goes:

3 BC   ...   2 BC   ...   1 BC   ...   AD 1   ...   AD 2   ...   AD 3   ...  

This is why it is incorrect to write "circa AD 0" for the start of the 1st century: there is no such year. Use "circa AD 1". (Traditionally AD appears before the year, though modern practice is not consistent.)

This means that the 1st century AD consists of the years AD 1 to AD 100 inclusive; the 2nd century consists of the years AD 101 to AD 200; etc. (The 1st century BC consists of the years 100BC to 1BC.) Thus, the 20th century consists of the years 1901 to 2000, and the 2nd Millennium consists of 1001 to 2000.

Thus, the 21st century and the 3rd Millennium begin with 2001.

Of course, it would be quite possible to redefine the start of a century as the "round number" and define the 20th century as 1900 to 1999. However, this would mean that the 1st century AD and the 1st century BC would have only 99 years each. This seems too great an anomaly.

It is worth noting that the problem only arises because the centuries are numbered as 1st, 2nd etc. Decades are not numbered in this way, and thus there is no problem in defining "the Sixties" as 1960 to 1969.

In fact, it is generally accepted by all authorities that the true start of the 21st century is 2001. See, for example, the Royal Greenwich Observatory site, or the White House millennium site. However, it was decided to celebrate New Year 2000 as being more dramatic in terms of the numbers flipping over.The obvious solution is to celebrate both!

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Further notes on the Christian calendar:

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The Year AD:

Dionysius dated the birth of Jesus as 25 December 753 AUC (AUC being a previous system of dating, from the traditional date of the foundation of Rome). This year is 1BC: AD1 is the first year of the Nativity. The subject is complicated by a number of factors, including:

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The date of Christmas

The origin of the date 25 December for Christmas is not certain. One possible explanation is that it was thought that the Spring Equinox (then taken as 25 March) was the logical date for the creation of the world and thus also for the Incarnation of Christ. Thus the Annunciation was dated 25 March, and the birth of Jesus nine months later at 25 December. (Source: Encyclopaedia Britannica, on-line edition, <>, article "Church Year - Christmas", accessed 4 December 1999.) It has also been suggested that the date 25 December was attractive because it coincided with the pre-Christian Roman festival of the Unconquered Sun on the same day. It is also worth noting that the Emperor Constantine had been a devotee of the Unconquered Sun until his conversion to Christianity, and for a while seems to have thought that the two could be combined. (Also note that some European Christmas traditions come from the Roman festival of Saturnalia on 17 December.)

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BCE and CE dates

BCE and CE are sometimes used instead of BC and AD. That is, AD 1999 is the same as 1999 CE; and 55 BC is the same as 55 BCE. CE and BCE stand for "Common Era" (also sometimes read as "Current Era") and "Before the Common Era". They are considered by their proponents to be more sensitive to those who have reservations about the implicit Christian references in AD (anno domini, "in the year of the Lord") and BC ("before Christ").


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Calendars: days, months and years

Calendars make use of certain natural time periods:

There is however a problem: these natural time units do not fit together neatly. A synodic (lunar) month is slightly more than 29.5 days long. This can be dealt with by using months of 29 and 30 days alternately. But the tropical year is a little more than 365 days, which gives 12 and a bit months.

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Using days, months and years

If we want to use all three astronomical time periods (day, month and year) some complicated adjustments have to be made to synchronize them.

Most years will have 12 months, but some will have 13 - a "leap month". The process of adding the extra month is called intercalation. There are a number of ways of doing this. One is simply to wait until the calendar appears to be a month out (which can be judged by astronomical or seasonal events) and then add the extra month. Another is to find some regular cycle of 12 and 13 month years which will intercalate just enough months. The Metonic cycle uses a 19-year cycle in which extra months are added in years 3, 5, 8, 11, 13, 16, and 19, thus producing 235 months in 19 years.

Such calendars have been used in a number of different cultures around the world. The Jewish religious calendar is of this type, using the Metonic cycle. Agricultural societies have often used such calendars because the month is easily observable, while the seasonal year is vital to farming activities. Traditional Tswana society apparently used a lunar calendar, with thirteen named months. It it is not, as far as I know, in use now, but the names of twelve of the months have been transferred to the 12 months of the western calendar. For a discussion of this calendar see
Bruce S. Bennett, "Intercalation in the Traditional Setswana Calendar", Botswana Notes and Records, vol. 50, 2018, pp. 18-31.
The traditional Chinese calendar, used for setting the dates of festivals, is also a lunar/solar calendar of this type although for official purposes China now uses the Gregorian calendar.

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Using days and years

However, the modern western calendar does not use lunar months. Instead, it uses arbitrary "months" which are not connected to the real phases of the moon. Thus, it uses only two of the natural astronomical time units, the day and the year.

This calendar is usually seen as starting with the Roman Julian calendar, though in some ways its ultimate ancestor is the Egyptian calendar. The Romans had until the 1st century BC used an exceptionally irregular calendar. In 46 BC Julius Caesar introduced a new calendar designed by the astronomer Sosigenes. The year was 365 days long with an extra day every fourth year, and was divided into twelve arbitrary "months". This was to a large extent taken from the ancient Egyptian calendar which used twelve arbitrary months and a 365-day year (though without the leap year). (Reference: see article on history of the calendar)

(Note: the Julian calendar must not be confused with the Julian period, which is a system used by astronomers for numbering days.)

This was very satisfactory for a long time, but unfortunately the year is very slightly shorter than 365.25 days, and so by the 16th century AD the year was beginning to get out of synchronization with the seasons. This was rectified by Pope Gregory XIII who in 1583 introduced the present form, known as the Gregorian calendar, under which three leap years are omitted in each 400 years. (Leap years are omitted in years divisible by 100 unless they are also divisible by 400; thus 1900 was not a leap-year but 2000 was.) Gregory also removed the surplus days that had acculumated: the day after 4 October 1582 was 15 October 1582. The days of the week, however, were not interefered with.

The Gregorian calendar was adopted by different European countries at different times, so that for a long time there were both "Old Style" (Julian) and "New Style" (Gregorian) dates in use. Russia was still using the Julian calendar in 1917, which is why the October Revolution happened in November (i.e. it was October Old Style, November New Style).

A quite different day/year calendar was used in the French Revolution from 1793 to 1806. Years were numbered from 1792 when the First Republic was declared. There were twelve non-lunar months of 30 days, followed by five extra days. The calendar, which replaced 7-day weeks with 10-day "decades", was part of the de-Christianization policy. Conversion tables can be found in books on the French Revolution.

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Using days and months

The Muslim calendar, by contrast, uses the lunar month but not the solar year. Months are of 29 or 30 days, kept in line with the phases of the moon, but there are always twelve months to a year. Thus a year is either 354 or 355 days long, and the months thus move through the seasons over time.

The years of the Muslim calendar, dated "AH", are thus not the same as western years and one cannot convert from one to the other simply by adding or subtracting a number. For an on-line converter see from which the following examples are taken.

31 December 2000 AD = 5 SHawwal 1421 A.H.

This appears to give a difference of 2000-1421 = 579 years. But

31 December 1800 = 14 SHa`baan 1215 A.H.

which would give a difference of 585 years.

Conversion tables can be found in books on the history of Muslim regions.

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The week

The week is a cycle of seven days which is never adjusted to coincide with anything - i.e., it is a part of the calendar which has always operated in isolation from the other parts. Seven does not go neatly into the length of either the lunar month or the solar year, and so any given date will fall on different days of the week at different times. The same week is observed by the Jewish, Christian and Muslim religions despite the differences in the other parts of their calendars. Interestingly, it was in use in the Roman Empire even before the rise of Christianity.

International standard ISO-8601 provides a method of numbering weeks in a year, counting Monday as the first day of the week. This convention is reasonable for business purposes as it keeps the two days of the weekend together, but the real first day of the week is Sunday, and has been so since ancient times, and the habit of using the ISO week in calendars for ordinary social use should in my view be deprecated.

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Addendum 2021

Proleptic calendars

A proleptic date is the date, according to some calendar, before that calendar was in operation. For example, suppose you want to refer to a precise date in 1000 BC. You might refer to it as, say 12 February 1000 BC by the Gregorian calendar, even though no-one at that time would have used such a date, since it is according to a system that did not then exist. A common use is to deal with the period when some of Europe was using the Old Style calendar and some were using New Style—you can refer to dates in Old Style countries by proleptic New Style dates.

AUC dates.

An ancient system counting from supposed date of the foundation of Rome, also abbreviated a.u. 1 AUC = 753 BC; 753 AUC = 1 BC; 754 AUC = 1 AD. This is sometimes still used for ancient dates.

BP dates, carbon-dating etc.

BP dates are used in connection with carbon-dating. Years BP indicate the number of years "Before Present" according to carbon-dating, the "Present" being defined for this purpose as 1950. Thus 40,000 BP means about 40,000 years ago. This is obviously not suitable for precise historical dates, but is used, especially in Archaeology, for periods calculated by scientific techniques, principally Carbon-14 (also "radio-carbon", "radiometric", 14C) dating.

Carbon-dating is a scientific process for determining the age of an object. There are two complications with such dates:

  1. The process only tells you approximately how old the object is. Thus a date has to be given with a standard deviation, giving a range of possible dates.
  2. It has been discovered that the ages given by the process are not quite right for earlier periods, and need to be adjusted or calibrated. The methods of calibrating or correcting the original answer, however, are still being perfected. Hence it is best to give both the uncalibrated and calibrated date, so that a later researcher can always do the calibration again for herself.

Historians unfamiliar with carbon-dating should seek advice from archaeologists when using such data.

Also note "mya" dates: The abbreviation "mya" stands for "million years ago". Hence 1.5 mya = 1.5 million years ago. This is used by archaeologists for Early Stone Age sites.

The Martian calendar

In recent times, scientists have needed a calendar for Mars. How will you keep track of a Mars rover? You can of course simply use Earth time, GMT, but this will have no relation to the days and nights on Mars, which are not the same length as on Earth, so you want some way of referring to the days and night. There have been different schemes, but so far there seems to be agreement only on year numbers and the nature of days. (The Martian year is roughly twice the length of the Earth year.) Years are dated from a great global dust storm in AD 1956, an essentially arbitrary date. The Martian solar day is 24 hours, 39 minutes and 35 seconds in Earth units, and the Martian day is known as a "sol" to avoid confusion with Earth days. Mars landers use "Local Solar Time", which is calculated directly from the position of the sun. This time is important for things like photography, in terms of camera angles etc. The Martian year has 668 sols (687 Earth days). Before Year 1 is Year 0, so the calendar has straightforward negative numbers without the BC/AD complication.

So far there does not seem to be an agreed scheme for months etc. within the year, though there have been a number of imaginative suggestions. A "month" would be even more arbitrary than in the Gregorian calendar, since Mars does not have a large moon marking out a long unit of time. Instead, at present scientists use the position of the sun, expressed in astronomical terms to express the point in the year. But if there is ever a human presence on Mars something more convenient may be needed.

By Bruce Bennett, email bennett@mopipi... [Click here for full email address]

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Copyright © Bruce Bennett 2000
Last updated 31 December 2000
(with only a few hours of the twentieth century left!)