Ab urbe condita (related with Anno Urbis Conditae: AUC or a.u.c. or a.u.) is a Latin phrase meaning "from the founding of the City (Ancient Rome)", traditionally dated to 753 BC. AUC is a year-numbering system used by some ancient Roman historians to identify particular Roman years. Renaissance editors sometimes added AUC to Roman manuscripts they published, giving the false impression that the Romans usually numbered their years using the AUC system. In fact, modern historians use AUC much more frequently than the Romans themselves did.
The dominant method of identifying Roman years in Roman times was to name the two consuls who held office that year. The regnal year of the emperor was also used to identify years, especially in the Byzantine Empire after 537, when Justinian required its use. Examples of continuous numbering include counting by regnal year, principally found in the writings of German authors, for example Mommsen's History of Rome, and (most ubiquitously) in the Anno Domini year-numbering system.
It's a specific subtrope of Alternative Calendar.
Examples of this trope:
- Ultima, the sequel to Proxima by Stephen Baxter, prominently features a spaceship run by Space Romans. As a result, the dates in chapter headings are given in both AD and AUC years (e.g. the book begins in 2227 AD and 2980 AUC).
- Roma Eterna, an Alternate History novel by Robert Silverberg, features a Roman Empire that endures at least until the 20th century. Its chapters are preceded by dates in AUC style (the latest is AUC 2723, that is 1970 AD).
- One Nation, Under Jupiter: How all the years are numbered, as Christianity never became the dominant religion.
- The video game Europa Universalis: Rome uses this type of calendar regardless of what nation you choose, even if you play as Carthage, Rome's Arch-Enemy.
- The game Imperator: Rome uses this, starting in 450 AUC, right around the time Rome started to grow in power. This is partly for engine-related issues — negative years/counting downwards can cause problems in Clausewitz (and it needs to have a single universal time, so no 'Year of the Consulship of X'), so AUC is useful in that has a Year One that is in the past from the start date yet is still relatively recognisable to a modern-day audience and is thematically appropriate to the game.