# Category:Centuries

century
unit of time lasting 100 years
Wikipedia
Instance ofunit of time
Subclass oftime interval,
set of 100 (year)
Part ofmillennium
Has part or parts
Measured physical quantity
• duration
Different from
• Wiek
Authority control

In the traditional notation for common calendars (including the Gregorian or Julian calendars), AD and BC years are both counted positively and inclusively (from the same epoch), but year order is reversed in BC since numbers are still positive and there's also no "year zero" in any period (there's also no "century zero" and no "millenium zero" in these common calendars):

• the Nth century AD starts in the base year (N×100 − 99) AD, and ends in year (N×100) AD — these AD years in the Gregorian calendar are the same in astronomic years which are strictly positive for this AD period;
• the Nth century BC starts in the base year (N×100) BC, and ends in year (N×100 − 99) BC — you need to substract 1 from the BC (positive) years in the proleptic Gregorian calendar, before changing the sign to get astronomic years which are negative (or zero) for this BC period ; astronomical year numbering does use a minus sign and includes a "year 0", so year 1 BC is equal to astronomical year 0, which comes just after year 2 BC equal to astronomical year -1, and so forth.

So for example:

• the 21st century AD ends on 31 December 2099 AD, but starts on 1 January 2000 AD (which also start of the 3rd millenium AD, itself terminated on 31 December 2999 AD);
• the 10th century BC ends on 31 December 901 BC, but starts on 1 January 1000 BC (which also starts the 1st millenium BC, itself terminated on 31 December 1 BC, i.e. at end of astronomic year 0).

Note that both the Gregorian and Julian calendars still did not exist in that BC period, but both calendars are commonly extended today so that their "proleptic" year numbers do not even match each other, and also do not match with the prior calendars using different epochs (the old Royal calendars, then the Republican Roman calendar still used at start of the Roman Empire until 42 AD when the Julian calendar was finally regularized). The Gregorian calendar started to replace the Julian calendar only after 1584 AD (at different dates depending on countries), and became the de facto worldwide standard (except for religious purposes) only during the 18th century AD, creating a shift with a dozen days skipped in the year where it was ordered (this difference continues to grow by one day at end of three of four centuries).

## Subcategories

This category has the following 88 subcategories, out of 88 total.