For the expanded world of Star Wars lore there has always been one point in the road that has functioned the same way for its calendar as we have one here on Earth. Just as the Gregorian calendar divides itself into "B.C." and "A.D." the Galactic Standard Calendar of Star Wars divided itself into "BBY" and "ABY," abbreviated to mean "Before the Battle of Yavin" and "After the Battle of Yavin." The integral third act of the original Star Wars became the linchpin of mapping out the entire galaxy far, far away. With the just released Star Wars: The Rise of Skywalker, Lucasfilm has made a pretty major change to this reading of the calendar though, but we have a way for you to update your records (and Wookiepedia).
In the pages of the newly released, Star Wars: The Rise of Skywalker: The Visual Dictionary, a new framing device for the calendar is revealing in the form of "BSI" and "ASI," meaning "Before Starkiller Incident" and "After Starkiller Incident." This means that the first firing of the weapon (not the destruction) on Starkiller Base from Star Wars: The Force Awakens is now the main focal point for the calendar instead of the conclusion of "A New Hope." The opening introduction for the new timeline in Star Wars reads:
"This is a record of history as it relates to the current galactic conflict. Events here are recorded in years relative to the Starkiller Incident -- the firing of the weapon that destroyed the Hosnian system, marking the start of open war against the First Order."
Because the events of The Force Awakens and The Last Jedi take place in succession, both are listed on Year "0" of the new calendar, even though the events of episode 8 (And much of episode 7) take place after the firing of Starkiller base even if just by a matter of days. A simple formula for converting anything on the Galactic Standard Calendar (anything on the BBY/ABY scale) into the new calendar is this: Is the year listed BBY? Add 34 to it. Is the year listed ABY? Subtract 34 from the number (though note negative numbers do not apply). Simple as that!
To make this even more confusing, there are even more calendars used in the galaxy far, far away including "The C.R.C. dating system" as revealed in 2018's Star Wars: Scum and Villainy: Case Files on the Galaxy's Most Notorious and the "Lothal Calendar" as referenced in the Star Wars Rebels calendars. We will not attempt to create a formula for these.
Star Wars: The Rise of Skywalker (currently the only thing listed on the 'ASI' part of the new calendar) is now playing in theaters.