Ab urbe condita
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 an specific subtrope of Alternative Calendar.