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After Landing was the initial dating system used on Pern. Events were dated based on how many years had passed since the colonists landed on Pern.
This dating system was based on the assumption that Threadfall lasted fifty years, with a two hundred year interval between Passes. However, this resulted in inaccuracies as not every pass was exactly fifty years due to the Red Star's erratic orbit. In response, a new dating system was introduced by AIVAS after it informed Pern's current leadership of the inaccuracies. This new dating system was known as a Aivas Adjusted Turn.