I am given to understand that 2016 AD is a part of the 21st Century.

Is it the case that 2100 AD is a part of the 21st century too? Or is it a part of the 22nd century?


3 Answers 3


From the New Oxford Dictionary:

Strictly speaking, centuries run from 01 to 100, meaning that the new century begins on the first day of the year 01 (i.e. 1 January 1901, 1 January 2001, etc.). In practice and in popular perception, however, the new century is held to begin when the significant digits in the date change, e.g. on 1 January 2000, when 1999 became 2000


How to count centuries:

''The 21st century will begin Jan. 1, 2001.'' That simple sentence, admitting no dispute, comes straight from the World Almanac and Book of Facts.

  • The Royal Greenwich Observatory agrees. So do the U.S. Naval Observatory, the Encyclopaedia Brittanica and Webster's Third International Dictionary, among others. And so do I.The starting date of the 21st century - and the third millennium - should not even be questioned.

  • The issue was settled to the satisfaction of everyone who could count straight a century ago, when Pope Leo XIII, Czar Nicholas II, President Charles William Eliot of Harvard and The New York Times all agreed that the 20th would begin with 1901.

  • Curiously, Kaiser Wilhelm and President Seelye of Smith College disagreed with them, and today there are millions, maybe billions, of misguided earthlings in that camp, thinking that the upcoming Big Day is Jan. 1, 2000.

  • The Savoy Hotel in London and the Rainbow Room in New York are already booked for millennial merry-making as the calendar flips from Dec. 31, 1999, to Jan. 1, 2000.

  • The self-anointed Millennium Society plans celebrations at midnight in each of the 24 time zones, and the biggest of all at the Great Pyramid of Cheops outside Cairo.

  • Yielding to no one, New York City aims to attract a million revelers to Times Square for the magic moment, as if all of Asia, Europe and Africa had not passed that point hours before.

  • They will all be a year too soon. Putting it simply, for the 20th century to end with 1999, the first century would have had to end with 99 and, to make it 100 years, it would have begun with the year zero. But there was no year zero.

An historical approach:

  • Indeed, the keepers of the calendar in ancient Rome did not even have a numeral for zero. There was no dispute about the end of the 1st century, because people had no idea that another century was about to begin. Centuries as we count them now were unknown.

  • The dating of years ''before Christ'' and ''anno Domini'' was not conceived until the 6th century, when Pope John I commissioned the monk Dionysius Exiguus, or Dennis the Little, to develop a system for setting the date of Easter.

  • At the time, the calendar was dated from the reign of the Emperor Diocletian, a fierce persecutor of the early Christians. So Dionysius switched the base to the date of Christ's birth.

  • Later historians established that Christ was born a few years earlier than Dionysius thought - in 5 or 6 B.C. rather than A.D. 1. That being so, the 21st century has already begun.

  • But that argument persuades neither the purists like me who insist on 2001 nor the odometer-fixated who want to see 1999 turn to 2000.

  • Robert Bartley, editor of The Wall Street Journal, said in a recent column that 2000 vs. 2001 is all wrong anyhow. By his reckoning, the 19th century began when Wellington bested Napoleon in 1815, the 20th began with the outbreak of war in 1914 and the 21st began when the Berlin Wall collapsed in 1989. At the least, Mr. Bartley bolsters the view that this whole business is rather arbitrary.

  • Michael Barkun, a scholar at Syracuse University's Maxwell School of Communications, offers as good a reason as any for believing that 2000 is the year to celebrate.

  • He calls it ''the unconscious tyranny that the decimal system exercises over our minds,'' our quasi-magical assumption ''that round numbers have a certain significance.''

  • Neal H. Ewing offered a practical reason in a letter to the editor of The New York Times on July 20, 1899:

  • ''The centurial figures are the symbol, and the only symbol, of the centuries. The initial figure 18 (remember, he was writing in 1899) is time's standard which the earth carries while it makes 100 trips around the sun. Then a new standard 19 is put up. Shall we wait now a whole year for 1901 at the behest of those who calculate by counting balls on an abacus?''

  • The debate in The Times went on for months. In an editorial on Dec. 8, 1899, Times editors pronounced themselves ''much disturbed'' by news the Kaiser had declared that the new century was about to begin.

  • In a second scolding a few days later, it said he ''must stand in solitary grandeur as the only man of any prominence who cannot count up to one hundred.''

  • In still another editorial, The Times made its position absolutely clear: ''Beyond question, '1899' means the one thousand eight hundred and ninety-ninth year of the Christian era, and the next to last year of the nineteenth century.''

  • The literary digest, in its final issue of 1899, joined the ranks for 1901. ''If there was a year 0,'' it asked, ''why not a century 000? Perhaps there are only 399 society leaders in the 400! Perhaps we should begin counting our ages one year later, making each of us a year younger!''

  • It concluded with ''one disquieting thought - that in a hundred years it will all be forgotten, and some 'letter to the editor' will start the whole whirl going again.''


  • 1
    Brings back the debates I had a little over 16 years ago as countless hordes of odometer-fixated party animals celebrated the beginning of the 21st century a year early :P
    – oerkelens
    Commented Jul 21, 2016 at 15:09
  • Having a separate popular and technical new century makes it easier to find a nice venue at a good price for the 2001 party. We were all at work the year before watching for the servers to all crash.
    – JDługosz
    Commented Jul 21, 2016 at 23:09

Strictly speaking, a {decade, century, millennium, &c} is any period of n years grouped in ns. So 1872 to 1881 is a decade, as is 1999 to 2008, and so on. However, when the calendar is introduced to modify what particular non-random semi-arbitrary decade is being discussed, there are other ways to verbally shortcut specific expected groups of years. For one example, "the 50s" are year groupings that are or end in 50-59, being then determined by the tens place, sometimes as a name of sorts. The Roaring Twenties being 1920 to 1929. Such periods also conveniently (yet rather just as arbitrarily) happen to be a decade.

If however the subject under consideration is a specific decade, as determined by the ones place grouping, then these verbal calendar construct decades begin on a 1 and end on a 10. The nth decade, counting.

Considering the calendar switch of BC/AD and BCE/CE, it's easy to get the first decade (1-10) but for the other method, impossible to get the 0s. That's of course because it isn't a thing, there is no 0-9. Decades surrounding that change would be more capricious than usual, -10 to -1 or -2 to 8 or -1 to 9 or 1 to 10 or 2 to 11. The second decade would either way be only 11-20 (second * decade = twenty) even though there is also a 10s, going from 10-19. Yet that 10-19 wouldn't be the second decade any more than 7-16 or 14-23 would be the second decade.

The first decade AD then is the first ten years, which is 1-10. Just like the first century is the first hundred 1-100, and the first millennium is the first thousand 1-1000. Giving them names this way linked to a calendar period counting from the start, that defines them, just like the forties means something and the eight-hundreds means something. Which neither is the same as saying the forth decade or the eighth century. They don't cover the same period, they aren't the same thing, they can't be compared like that.

Linguistically, any group of one hundred years is a century. However. Going by a century grouping bounded by century (naming it according to the hundreds) the 1600s can only be 1600-1699. Going by which century in a string of them (naming it according to the order of every hundred years since the starting one) the 16th century can only be 1501-1600. The sixteen hundreds isn't the seventeenth century, because they aren't even the same concept to begin with.

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