Is 2000 in the 20th or the 21st Century?

The year 2000 belongs to the 20th century. It is the century's last year, not the first of the 21st, which began on 1 January 2001. In Roman numerals, 2000 is MM and its century is XX.

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On this page
  1. Why 2000 closes the century
  2. The year that ends each century
  3. The quick test for any year
  4. The 2000s are not the 21st century
  5. Writing "2000" correctly
  6. Sources

Why 2000 closes the century

A century is a hundred years, and the count started at AD 1 with no year zero. The first century was AD 1 to 100, so the 20th is 1901 to 2000. NRICH, the mathematics project of the University of Cambridge, explains that 2,000 years "will only have elapsed" at midnight on 31 December 2000, which is why the 21st century and the third millennium began together on 1 January 2001 (NRICH).

Many people celebrated the new millennium on 1 January 2000, and NRICH acknowledges that. The celebration followed the change of digits from 1999 to 2000; the century count changed a year later.

The year that ends each century

Every century ends in a year ending in 00. Here are the last few, with their numerals:

CenturyRomanLast yearYear in Roman numerals
17thXVII1700MDCC
18thXVIII1800MDCCC
19thXIX1900MCM
20thXX2000MM
21stXXI2100MMC

Notice how short the closing years are in Roman numerals: no tens or units, just thousands and hundreds. MCM (1900) and MM (2000) are often mistaken for centuries because they look round, but they are years.

The quick test for any year

Divide the year by 100. If the result is a whole number, that's the century. If not, round up:

  • 2000 ÷ 100 = 20 → 20th century → XX
  • 2001 ÷ 100 = 20.01 → 21st century → XXI
  • 1900 ÷ 100 = 19 → 19th century → XIX

The 2000s are not the 21st century

"The 2000s" and "the 21st century" sound like the same thing, but they are counted differently. Read as the years whose first two digits are 20, the 2000s run from 2000 to 2099. The 21st century runs from 2001 to 2100. They overlap almost completely, except for one year at each end: 2000 is in the 2000s but not in the 21st century, and 2100 is in the 21st century but not in the 2000s.

The same goes for the 1900s (1900 to 1999) and the 20th century (1901 to 2000). In Roman numerals the difference is easy to see: the century is XX, while the years 1900 and 2000 are MCM and MM.

Writing "2000" correctly

  • The year: MM. Two M's, nothing else.
  • A date in 2000: 1 January 2000 is I.I.MM in day-month-year order.
  • The century it belongs to: XX, the 20th.

If you need the first year of the 21st century, it's 2001: MMI.

The 21st century in Roman numeralsIts first and last year, its decades in Roman numerals and how to write it in each language.

Sources

  1. When does the new millennium start?. NRICH, University of Cambridge