Star Wars: Rogue One: The Ultimate Visual Guide states that the Onderonian Civil War began the same year the Clone Wars began, which Star Wars: Timelines dates to 22 BBY. Therefore, the Onderonian Civil War began in 22 BBY as well. The Civil War's end, depicted in "Tipping Points," occurs in 20 BBY according to Star Wars: Timelines.
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{{OnderonianCivilWarDate}}