The 'ABY-BBY' dating system was used to label or number years in the galactic history for some time during the Galactic Civil War, mostly within the hierarchy of the Alliance to Restore the Republic. The "year zero" in this system was based on the destruction of the first Death Star during the Battle of Yavin, which the Alliance leaders saw as their first major victory against the Galactic Empire. BBY stood for "Before the Battle of Yavin," and ABY for "After the Battle of Yavin." Dates classed as 0 BBY and 0 ABY both denoted events that happened in the same year as the battle, either in the months before it, or the months after it.
The 'ABY-BBY' system was also used on a collection of maps thought to have been created by the Ithorian artist Gammit Chond. Various events related to the locations shown were marked on the maps with dates included, with many of the events centering on the exploits of the legendary Skywalker family.
Behind the scenes
The 'ABY-BBY' dating system was first used in Star Wars canon in 2014, in Star Wars Workbook: 2nd Grade Reading. In 2016, the reference book Star Wars: Galactic Atlas by Emil Fortune made extensive use of this system. It was originally introduced in the Star Wars Legends continuity in the 1996 West End Games roleplaying game guide Star Wars: The Roleplaying Game, Second Edition, Revised and Expanded written by Bill Smith.