The year36,453 before the Battle of Yavin (BBY) was a period of time in the pre-Republic era. That year marked the Tho Yor Arrival—the eight Tho Yor pyramid ships became active and traversed the galaxy, picking up Force-sensitive passengers of over a dozen species from many different worlds. The Tho Yor then traveled to the Deep Core world of Tython, where they gathered around the ninth and largest Tho Yor. The Tho Yor Arrival sparked an immense Force Storm, the strength of which would go unrivaled for almost ten thousand years, as Tython reacted to the presence of so many Force-sensitives. The Tho Yor then scattered themselves across Tython, and the pilgrims soon formed the Je'daii Order, a Force-using organization that strove to maintain balance between the light and dark sides of the Force in both themselves and the world around them.
↑Star Wars: Dawn of the Jedi Volume 2—Prisoner of Bogan establishes the year36,453 BBY as the date of the Tho Yor Arrival, which is the zero point of the Tho Yor Arrival dating system, meaning the difference between the calendar based on the Battle of Yavin and the TYA year-notation system is 36,453 years. This date is derived from simple math based upon other confirmed dates.
↑The New Essential Chronology states that the Ruusan Reformation occurred in 1000 BBY, which is the zero point of the calendar based on the Ruusan Reformation, so the difference between the calendar based on the Battle of Yavin and the Ruusan Reformation's year-notation system is 1,000 years. This date is derived using simple math based upon other confirmed dates.