Unlike many other fictional universes, Star Wars does not have an official calendar. Instead, the franchise's timeline has long been anchored to the climactic event from the original 1977 film: the Battle of Yavin and the destruction of the first Death Star. Since this reference event for the entire Star Wars timeline was established, time in years has usually been designated using BBY ("Before the Battle of Yavin") and ABY ("After the Battle of Yavin"). For example, Luke Skywalker was born in 19 BBY, during the events of Revenge of the Sith, and died in 34 ABY, during the events of The Last Jedi.
Although this dating system helps keep the chronology clear for fans, the absence of an in-universe calendar has led to some confusion as the universe continues to expand. After all, most calendars, fictional and not, do not reset after a significant historical event such as a battle. Star Wars has recently tried to rectify this issue by redefining its eras and using the Starkiller Base's strike on the Hosnian system from The Force Awakens as a new anchor point. However, Star Wars has also quietly introduced an additional new dating system that both simplifies and complicates the timeline even further.
Last fall, a reference book titled The Star Wars Book was released. Co-written by Pablo Hidalgo, Lucasfilm Stary Group executive and continuity expert, the book introduced new dating system that also uses the events of Episode IV: A New Hope. To replace BBY and ABY, the book's glossary defines the new designations BSW4 ("Before Star Wars 4") and ASW4 ("After Star Wars 4"):
"After Star Wars 4/Before Star Wars 4, a dating system used to describe the timeline of events in Star Wars stories. Indicates how many standard years before or after Star Wars: Episode IV - A New Hope an event takes place."
Now, the anchor point of the chronology of Star Wars is no longer tied to a single battle, but more generally the events of the first movie in the series. Although this new designation might make it easier for fans who may not be fully versed in every detail and battle of the Star Wars universe, it still confuses things once again.
Anchoring everything to the general events of Episode IV doesn't quite solve the problem that the Battle of Yavin designation causes, as it still focuses on generally the same point in time as BBY/ABY did. Hinging everything on certain battles and/or movies might make sense for fans who have extensive knowledge of every story detail, but the practice is becoming increasingly arbitrary as Star Wars continues to engage in multi-media storytelling.
By avoiding creating an in-universe calendar, Lucasfilm story executives have created even more uncertainty as to which designation to use. With the added baggage of the BSI/ASI (Before and After Starkiller Incident) and the new Era designations, it puts both fans and creators at a confusing disadvantage.
As the universe continues to expand its scope and tell new stories through different mediums, anchoring chronology to notable fictional events is become increasingly untenable. Perhaps it is high time for the Story Group to officially come together and create the Star Wars version of the Gregorian Calendar.