Star Wars Timeline of Books

Fans of the Star Wars fictional universe keep track of the dates of key events with a dating system (or calendar) reckoned at the Battle of Yavin featured in Star Wars Episode IV: A New Hope. In this dating system, BBY stands for Before the Battle of Yavin, and ABY stands for After the Battle of Yavin, so 50 BBY would come before 5 ABY, and 45 BBY would come after 50 BBY, much as in AD and BC. (Occasionally, the defeat of the Empire at the Battle of Endor (four years after Yavin) has been used as an alternate starting year, but has generally been phased out in favor of the current standard system. When Bantam Spectra held the license for Star Wars novels, they described their books as after Endor, but Del Rey, the current license-holder, exclusively uses ABY.) Source

Note: In-universe, this dating system is used almost exclusively by the Rebel Alliance/New Republic. The earlier Galactic Empire preferred to date events from the Great Resynchronization of 35 BBY, or from the Emperor's ascension to the throne (19 BBY). Originally, the Old Republic dated years from the Ruusan Reformation of 1,000 BBY.

A galactic year, based on the length of a year on Coruscant, is 368 standard days, one standard day being a time period of equal length as a day on Coruscant (twenty-four hours). It is composed of 10 months.