Century

Start and end in the Gregorian Calendar

According to the Gregorian calendar, the 1st century AD/CE started on January 1, 1, and ended on December 31, 100. The 2nd century started at year 101, the 3rd at 201, etc. The n-th century started/will start on the year (100 × n) − 99 and ends in 100 × n. A century will only include one year, the centennial year, that starts with the century's number (e.g. 1900 is the final year of the 19th century).

Debate over century celebrations

Because the Gregorian Calendar does not have a year 0, purists have argued that a new century does not begin until **01.

Dionysius Exiguus of Scythia Minor introduced the anno Domini system in AD 525, counting the years since the birth of Christ.^[10] This calendar era is based on the traditionally reckoned year of the conception of Jesus of Nazareth, with AD counting years from the start of this epoch, and BC denoting years before the start of the era. There is no year zero in this scheme, so the year AD 1 immediately follows the year 1 BC. This dating system was devised in 525, but was not widely used until after 800.

There is a year 0 however in the astronomical year numbering and in the ISO 8601:2004. In this case, both 1900s and 20th century have the same time span (1900-1999). However, most people assume and prefer that a new century begins with two 0s as its last digits (1700, 1800, 1900, 2000, etc.)

Therefore the 1st century spans from 1-100, the 2nd century spans from 101-200, the 20th century spans from 1901-2000, and the 21st century spans from 2001-2100. There is an opposite way to count centuries from **00 to **99 all together. For example the 1900s century began on January 1, 1900 and ended on December 31, 1999. The 2000s century began on January 1, 2000 and will end on December 31, 2099. Therefore the year 2000 is the last year of the 20th century, it is the first year of the 2000s century.

1st century BC and AD

There is no "zeroth century" in between the 1st century BC and the 1st century AD. Also, there is no year 0 AD. The Julian calendar "jumps" from 1 BC to 1 AD. The first century BC includes the years 100 BC to 1 BC. Other centuries BC follow the same pattern.

Dating units in other calendar systems

Besides the Gregorian calendar, the Julian calendar, the Aztec calendar, and the Hindu calendar have cycles of years that are used to delineate whole time periods; the Hindu calendar, in particular, summarizes its years into groups of 60, while the Aztec calendar considers groups of 52.

Centuries in astronomical year numbering

Astronomical year numbering, used by astronomers, includes a year zero (0). Consequently, the 1st century in these calendars may designate the years 0 to 99 as the 1st century, years 100 to 199 as the second, etc. Therefore, in order to regard 2000 as the first year of the 21st century according to the astronomical year numbering, the astronomical year 0 has to correspond to the Gregorian year 1 BC.

Alternative naming systems

In Swedish, Danish, Norwegian, Icelandic and Finnish, besides the ordinal naming of centuries another system is often used based on the hundreds part of the year, and consequently centuries start at even multiples of 100. For example, Swedish nittonhundratalet (or 1900-talet), Danish nittenhundredetallet (or 1900-tallet), Norwegian nittenhundretallet (or 1900-tallet) and Finnish tuhatyhdeksänsataaluku (or 1900-luku) refer unambiguously to the years 1900–1999.

The same system is used informally in English. For example, the years 1900–1999 are sometimes referred to as the nineteen hundreds (1900s). This is similar to the English decade names (1980s, meaning the years 1980–1989).