The galactic standard calendar, also known as the galactic calendar, was in use sometime after the Battle of Yavin during the Galactic Civil War that used the pivotal battle as a zero year from which to record galactic history. Dates following the pivotal battle were marked as "ABY" while dates beforehand were marked as "BBY".
The galactic standard calendar came into use sometime in the New Republic Era. The calendar was 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 from the calendar included, with many of the events centering on the exploits of the legendary Skywalker family. Chond never visited any of the locations he was thought to have drawn, and was believed to have based the information they contained on travelers' tales, as he never left his homeworld.
Behind the scenes
The galactic standard calendar was first used in Star Wars canon in 2015 on the Star Wars: Card Trader card "R2-D2 - Jabba's Sail Barge," which described the year of the rescue of Han Solo from Jabba the Hutt as 3 ABY. In 2016, the reference book Star Wars: Galactic Atlas by Emil Fortune expanded on the meaning of the BBY/ABY dating system and gave its name as the galactic calendar. 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.