Galactic calendar

A calendar system was a measurement of time in the galaxy. It centered around the planet Coruscant's solar cycle. Numerous epochs were used to determine calendar eras; however, this system of calendar eras used the Battle of Yavin as its epoch, or "year zero." BBY stands for Before the Battle of Yavin, and ABY stands for After the Battle of Yavin.

History
The galaxy's earliest recorded events recorded by this calendar occurred in 3966 BBY.

A similar dating system, the Lothal Calendar, was first recorded to be used in 3277 BBY.

Behind the scenes
BBY/ABY is also known as Before A New Hope/After A New Hope, meaning "before/after Star Wars Episode IV".

Star Wars: Galactic Atlas was the first source to use the BBY/ABY in canon.