Anno Koyomi was the first dating convention used by Yukito Kishiro to place the events of Battle Angel Alita within a timeline, relating them in relation to the year of Koyomi's birth. It was superseded by the Era Sputnik convention, which allows for much greater flexibility.
Anno Koyomi was introduced with the Gunnm: Another Stories side stories in order to relate how they fit within the overall Battle Angel Alita story. Treating the year of Koyomi's birth as Year 0, events taking place before this were given the prefix "B.K." (Before Koyomi) while events taking place after were given the prefix "A.K." (Anno Koyomi), with the corresponding number of years before or after. This first revealed that Daisuke Ido's discovery of Alita in the Tiphares scrapheap and Koyomi's birth occurred in the same year, AK 0.
After Era Sputnik (ES) was introduced, later timelines indicating where each side story occurs were revised to reflect the transition to ES for the sake of uniformity. A comprehensive ES timeline appears in the collected version of Another Stories. Here Koyomi's birth and Alita's discovery are set as having occurred in ES 577.