G's research finally turns up a slim volume in the Forbidden Knowledge section titled "A Visit to Arvendor" -- by Streck Brightblade. It appears that the Nirduk calendar is synchronized with Streck's own personal time clock. G spends some time reading the book:
Apparently Streck was struck by a prismatic spray attack and sent to a forested plane inhabited by elves and orcs. The orcs were led by a powerful and vicious one-eyed king who wielded a flaming spear. In this world, the elves were poorly equipped and non-magic using. They welcomed Streck as an avatar from beyond. He spent many decades with them, teaching them about swords and spells, and finally led a glorious battle against the orcs. Streck fought and killed the Orc king in a great battle and found a gate home. It was then that Streck saw the parallel between his deeds and those legends of Corellan that he had heard as a child. He realized that he was actually an incarnation of Corellan and has devoted his life to promoting the elven way.
Nomistrus is not sure about the validity of this book, but it does give an explanation for why the Nirduk calendar is off by about 50 years.