Galactic Standard Calendar

The Galactic Standard Calendar was the standard measurement of time in the galaxy. It centered around the Coruscant Solar cycle, which was 368 days long (one day consisting of 24 standard hours). Numerous epochs were used to determine calendar eras. The most recent of these 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 calendar, also referred to as the Coruscant Standard Calendar, was the main calendar in use in the galaxy since the time of the Galactic Republic. Presumably the Old Republic dated years from its founding in 25,053 BBY. Throughout time, however, historians have used numerous galaxy changing events as epoch to mark new calendar eras.

One particularly notable epoch is the Treaty of Coruscant of 3,653 BBY. The calendar eras before and after this event (referred to as "BTC" and "ATC," respectively) were popularized by the famous Jedi historian, Gnost-Dural. His holographic records, which used this numbering system, contained some of the most complete records of numerous important 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 War against the returned Sith Empire. For this reason, this method of numbering years remains important to historians.

Other notable epochs used were the Ruusan Reformation of 1,000 BBY,, the Great Resynchronization of 35 BBY, the formation of the Galactic Empire in 19 BBY and the Battle of Endor in 4 ABY. With the exception of the Ruusan Reformation, the latter epochs were all within the same century and stemmed from the events and upheavals surrounding the rise and fall of the Galactic Empire. They were short lived and used by various historians at the time. In 25 ABY, the New Republic commissioned the New Republic Historical Council to re-standardize the Galactic Calendar. The historical council chose the Battle of Yavin, instead of the Battle of Endor, calling the former the more significant galactic event. From that point on, the year in which the Battle of Yavin occurred was the epoch used for the dating system. It was used by the New Republic, and the subsequent Galactic Alliance. Many regions, however kept their own calendars, including the Imperial Remnant.

Time measurement
The calendar was based on the size and rotation of the planet Coruscant. It was a luno-solar calendar based both on Coruscant's orbit around its sun, and the orbit of its primary satellite Centax 1. The standard unit of time was the standard second. Sixty standard seconds made up each standard minute, and sixty minutes made up each standard hour. Each day consisted of 24 standard hours. A standard week was 5 standard days, and each month was seven weeks. (Making 35 standard days each month.) A standard year was 368 days, composed of ten months, three fete weeks, and three holidays. As the Hyperdrive Theory allowed space travellers to bypass relativity, a single duration of time passed at all locations equally over a given interval.

In the galaxy:


 * 368 days=1 year
 * 60 seconds=1 minute
 * 60 minutes=1 hour
 * 24 hours=1 day
 * 5 days=1 week
 * 7 weeks=1 month
 * 35 days=1 month
 * 10 months + 3 festival weeks + 3 holiday days=1 year

Behind the scenes
BBY/ABY is sometimes known as BSW4/ASW4, which means "before/after Star Wars Episode IV". The BSW4/ASW4 notation was used 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 eschewed any acronyms altogether and listed dates as YEARS, with 0 YEARS being the events depicted in Star Wars Episode IV. The first Essential Guide to use the BBY/ABY notation was The Essential Chronology. This practice was continued in The Essential Guide to Alien Species and beyond. Despite its fall into disuse within official materials and the use of the BBY/ABY notation by most of their peers, the early method of notation has caught on a small following among Star Wars fans.

When the Thrawn trilogy was released, most of the work in the Expanded Universe (promotional material originally dated events in terms of the Battle of Endor) made sense from an in-universe perspective of having "overthrown the Emperor" rather than victory at the Battle of Yavin. This trailed off some time after the release of The Truce at Bakura.

The video game Star Wars: The Old Republic uses the Treaty of Coruscant as its dating system. Since the date of the treaty is stated to be 3,653 BBY it makes some of the dates listed on Timeline 1: Treaty of Coruscant up to 53 years off when converted to the Yavin dating system.

Time based on Coruscant is clearly a mirroring of real Earth time being modeled on the third planet from the sun.

Thursday, Saturday and Sunday have all also been mentioned as days in the EU.

It is known that the beginning of the in-universe calender does not correspond with BBY/ABY as in Fury. It is stated that a new year has begun yet the book still takes place in 40 ABY just as the other books in Legacy of the Force do.