The Treaty of Coruscant marked the end of the Great Galactic War between the Galactic Republic and the Sith Empire. The two factions met on Alderaan to discuss the terms of peace, while the treacherous Sith launched a simultaneous attack against the Republic on their home world of Coruscant. The sacking and occupation of Coruscant allowed the Empire to force the Republic into a completely helpless position, leaving them no choice but to surrender to the Empire's terms. The Republic gave up control of multiple star systems and trade routes to the Sith Empire and gained little to nothing in return. This treaty marked the beginning of a Cold War.
All dates were adjusted to a new time system after the treaty was signed. So every event that takes place after the treaty would be ATC (After Treaty of Coruscant) while the events that have happened before would be BTC (Before Treaty of Coruscant). For example, the game itself takes place at 10 ATC, as stated in the Blood of the Empire web comic on the main site.