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Wikipedia says that

The 21st century began on January 1, 2001 and will end on December 31, 2100.

The first decade of 21st century is the 2000s. Here Wikipedia says that

The 2000s was a decade that began on January 1, 2000, and ended on December 31, 2009.

I don't understand. If the 21st century starts from 2001, then how can it be that the first decade of the 21st century starts from 2000?

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    Might be better suited to English language and useage? I am not sure I understand how this is history. – Mark C. Wallace Jul 20 '18 at 16:01
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A decade is simply a time span of 10 years and a century a span of 100 years. The start dates of each are determined by how they are being used.

The first decade of 21st century is 2000s

I think that statement is where the problem lies. Strictly speaking it isn't true.

Under the Gregorian calendar, the 21st Century started on January 1, 2001. The date is fixed by the fact that there was no year zero. So every century within that calendar started on January 1, XX01.

As far as decades within centuries, it's a matter of how they are being used. Most people would interpret the "2000s" as being January 1, 2000 to December 31, 2009 because they would interpret 2010s to be from January 1, 2010 and ending on December 31, 2019, etc.

However, if you were talking specifically about the first decade of the 21st Century you'd be discussing January 1, 2001 to December 31, 2010.

  • Will I consider decade as January 1, XX01 to December 31, XX10 ? – user149054 Jul 20 '18 at 11:39
  • @user149054 If you are talking specifically about the first decade of a given century then, yes, it's January 1, XX01 to December 31, XX10. Depending on context it may be that considering a decade of January 1, XX00 to December 31, XX09 is just as useful or even the same (especially if the defining events of the decade occur in the middle years). – Steve Bird Jul 20 '18 at 12:07
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    @user149054 Technically, it's incorrect for the reasons given in the answer. Whether it actually matters or not is a question of context. If you include the date range then there's no ambiguity. If you just say "the first seven years of the new millennium" then there's scope for different interpretations. – Steve Bird Jul 20 '18 at 13:03
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    One additional point is that "a century" is also "simply a time span", so it's not quite true that "every century started on January 1, XX01", only that the conventionally numbered centuries started on those dates. Just as there is a decade from January 1, 2000 to December 31, 2009, there is a century from January 1, 2000 to December 31, 2099; it just isn't what is conventionally called the "21st Century". "The 2000s" is using a different naming scheme from "the 21st Century", so there's no reason the two have to begin on the same date. – IMSoP Jul 20 '18 at 13:40
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    @IMSoP Agreed, I've amended the answer to reflect that. – KillingTime Jul 20 '18 at 14:04
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You're conflating mathematical precision with linguistic labeling.

The first decade starts on January 1 of a year ending with a "0".

The problem stems from the fact that when our dating system was created people commonly used the reigns of kings and leaders to describe events. We are now in the second year of the reign of Trump. In the 6th year of the reign of Obama the Russians invaded Crimea.

The same thinking was used by to create the current calendar. We are now in the Year of our Lord (Anno Domini) 2018. We changed AD to CE but we kept the numbering system.

Of course, in such a numbering system there is no year 0, hence causing our problem when we try to conform our naming system with more precise mathematical rules.

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    No, not the reign of trump, but the 65th year of the reign of Queen Elizabeth II... – Solar Mike Jul 20 '18 at 18:00
  • The first decade of the first century CE began on 1 January 1 CE. That doesn't end with a "0". – sempaiscuba Jul 22 '18 at 0:45
  • @sempaiscuba - you're taking a retrospective mathematical approach to the situation. If we had the same counting method we would now be in the second year of the reign of Trump. There is no year zero. – Mayo Jul 26 '18 at 12:59
  • I'm not sure what point you're making. Since Trump was inaugurated on 20 January 2017, we are in the second year of his Presidency. However, since the President doesn't rule as a monarch (something made clear in The Constitution), it would be wrong to call it the second year of his reign. – sempaiscuba Jul 27 '18 at 1:56
  • The point has to do with understanding the counting system used over millennia. It was based on the reign of the ruler. There was no year zero. So, when Dionysius Exiguus was creating a calendar system he used the same concept. The year 1 started the year that Jesus of Nazareth was born. The Trump and Obama references was to help the reader think in those terms. We're adding mathematical precision to a system that did not have a year zero. The simplest was to go along with this is for the first "century" to have 99 years. – Mayo Jul 27 '18 at 12:46

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