In-Universe Calendars

The primary system of dating events for the Star Wars universe is by measuring how far it took place from the Battle of Yavin (Year Zero), the setting of the first Star Wars film and a turning point in the rebellion against the Galactic Empire. This method is taken as one of many in-character ways of measuring years, however, it only makes sense for events set following the battle.

Most of the galaxy uses the 'Coruscant Standard Calendar' based around the solar and orbital cycles of Coruscant (24 hours per day with 368 days per year). A Coruscant week consisted of five days: Primeday, Centaxday, Taungsday, Zhellday and Benduday. Seven days made up a month, and depending on the calendar, ten or eleven months made up a year.

Year systems change much more frequently. While the ABY/BBY year system does not work for any in-character interactions prior to the Battle of Yavin, other significant historical events are often used to mark eras to refer to years by.

The Treaty of Coruscant

The end of the Great Galactic War and the signing of the Treaty of Coruscant was a major point of change for the Republic. With Coruscant attacked, the Republic was forced to accept the treaty, which meant recognizing the Sith Empire as the legitimate owner of half the galaxy. Following this, the Republic no longer stood as the dominant power in the galaxy, having been defeated militarily by the Empire and forced to concede much of their territory.

Use of the Treaty of Coruscant as a marker year was popularized throughout Republic space by Jedi historian Gnost-Dural. In this system, the year of the treaty, 3653 BBY, is Year Zero and any year after it is denoted as ATC (After the Treaty of Coruscant) and before it as BTC (Before the Treaty of Coruscant). For example: the Battle of Alderaan took place in 3667 BBY, 14 years before the treaty, is 14 BTC. The breaking of the treaty and the renewal of war was in 3642 BBY, 11 years after the treaty, is 11 ATC.

Years of the Empire

Unlike the Republic, the resurgent Sith Empire can trace its founding to a relatively recent date, which is indisputably one of the most important moments in the history of the Empire. In 4980 BBY, the Sith Emperor's fleet, consisting of the remnants of Naga Sadow's Empire, landed on Dromund Kaas after twenty years of wandering. It was then that the Emperor declared his new regime, and the Sith Empire was reborn.

The use of Years of the Empire (YE) is enforced across the Empire and all documents are required to subscribe to this format. However, the days of the week and length of the year still follows the Coruscant Calendar due to the populace being accustomed to it despite the best attempts of Imperial officials to convert to a system based around the orbit of Dromund Kaas.

The Imperial calendar does not have a Year Zero. Instead, 4980 BBY is counted as the First Year of the Empire, or 1 YE, and one is added for every subsequent year. Any year prior to that would be counted as a year Before the Empire; for example, 4981 BBY is 1 BE. To calculate a year before the empire, simply subtract the year from 4980. To calculate the Year of the Empire, subtract 4980 by the year in question and then add 1. 0 ATC or 3653 BBY, the year of the Treaty of Corusant, would thus be 4980-3653+1, which is 1328 YE.

Addendum for easy reference: the events of SWtoR in game begin in 3643BBY / 10ATC with the cold war. This translates to 1338YE.