Star Wars: The Old Republic Encyclopedia places the Treaty of Coruscant, which served as the zero point of a calendar adopted by the Galactic Senate following the conclusion of the Great Galactic War per Galactic History on The Old Republic's official website (article) (backup link), in the year 3653 BBY of the Galactic Standard Calendar, which was based around the date of the Battle of Yavin. Thus, the difference between the two dating systems is 3,653 years.