↑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.
↑Andur Sunrider leaves H'ratth for Ambria in 4,000 BBY per Tales of the Jedi 3: The Saga of Nomi Sunrider, Part 1. The Tales of the Jedi Companion says that 2 years prior (4,002 BBY), Andur went to H'ratth, and six years prior to that (4,008 BBY), he saw the Force ghost of his grandfather Jev, who the Companion says disappeared on a mission a year prior to that (4,009 BBY). If you start at 4,000 and count back the number of years (2, then 6, then 1) you arrive at 4,009 BBY, which has to be the year of Jev's death, given that he appeared to Andur in ghost form a year later (4,008 BBY).