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Prior to this time, "After Landing" was the dating system used on Pern. It was based on the assumption that every Threadfall lasted fifty years and Thread fell every two hundred fifty years.
Events were dated based on how many years had passed since the colonists landed on Pern.
In the Ninth Pass, AIVAS reported that not every fall had lasted fifty years. The new dating system, Aivas Adjusted Turn, was implemented.