The standard way of measuring time across the galaxy was known as the Galactic Standard Calendar, also called Galactic Standard Time. Its foundation was the solar cycle of Coruscant. This cycle on Coruscant lasted 368 days, with each day divided into 24 standard hours. To define different calendar eras, various epochs were employed. Of these, the most recent used the Battle of Yavin as its starting point, or "year zero." The abbreviation BBY signifies Before the Battle of Yavin, while ABY means After the Battle of Yavin.
Galactic Standard Time, often shortened to GST, also referred to a specific time zone used throughout the galaxy, which could be tracked using a chronometer. GST midday occurred at the twelve-hour mark of a standard day, written as 12:00 GST.
Throughout history, numerous pivotal events that reshaped the galaxy have been used by historians as epochs to start new calendar systems.
A particularly important epoch is marked by the Treaty of Coruscant in 3653 BBY. The periods before and after this treaty, known as "BTC" and "ATC" respectively, gained popularity thanks to the renowned Jedi historian Gnost-Dural. His holographic records, which utilized this dating system, contained comprehensive accounts of significant events such as the Hundred-Year Darkness, the Great Hyperspace War, the Great Sith War, the Mandalorian Wars, the Jedi Civil War, and the Great Galactic War against the returning Sith Empire. Consequently, this method of dating years remains relevant to historians. Millennia later, the Ruusan Reformation in 1000 BBY significantly reorganized the Galactic Republic, establishing a new calendar with 1000 BBY as its starting point. Centuries later, in 36 BBY, the Republic Measures & Standards Bureau introduced the Great ReSynchronization calendar to harmonize the various dating systems in use by the Galactic Republic.
Finally, around 25 ABY, the New Republic Historical Council created its own dating system, selecting the Battle of Yavin as the more important galactic event over the Battle of Endor. From that moment forward, the year of the Battle of Yavin became the epoch for their dating system. The New Republic's calendar remained in use during the Sith–Imperial War.
- 60 standard minutes equals 1 standard hour [4]
- 24 standard hours equals 1 standard day [4]
- 5 standard days equals 1 standard week [4]
- 7 standard weeks equals 1 standard month [4]
- 10 standard months plus 3 festival weeks plus 3 holidays equals 368 standard days, which equals 1 standard year [4]
In 23 BBY, the Republic Measures & Standards Bureau discussed whether to continue recognizing this calendar as one of the Republic's approximately twenty official calendars. Keelen Ma noted that researchers were growing weary of needing calendar converters on their data pads.
BBY/ABY is sometimes referred to as BSW4/ASW4, which is short for "before/after Star Wars: Episode IV." The BSW4/ASW4 system appeared in the timeline of the second edition of A Guide to the Star Wars Universe and The Essential Guide to Vehicles and Vessels. The Essential Guide to Planets and Moons avoided using acronyms altogether, instead listing dates as YEARS, with 0 YEARS representing the events of Episode IV. The first Essential Guide to adopt the BBY/ABY system was The Essential Chronology. This practice continued in The Essential Guide to Alien Species and subsequent publications.
In 2005, Leland Chee, the keeper of the Holocron continuity database, revealed that the 10-month calendar system initially present in the Expanded Universe was discontinued in later works and replaced with a more practical 12-month calendar. Sue Rostoni later confirmed the use of a 12-month/368-day calendar. Consequently, The Essential Atlas featured years consisting of 12 months, with months having 30 or 31 days, as did The New Essential Chronology. However, Karen Traviss stated that she utilized the 10-month calendar while writing her Republic Commando novels, even though the one-year anniversary of the First Battle of Geonosis occurs roughly twelve months after the battle in those novels.
Star Wars: New Planets, New Perils! mentioned Sunday school, but did not explicitly state that it was named after a day of the week.