8th Secret Craft of Mystara : Chronomancy
by Marc SaindonI posted way back that the old TSR supplement Chronomancer mentions Mystara). One idea how to use this in a campaign is that the Divinarchy of Yavdlom, which is known to train Divination Wizards, could have a secret craft like the Glantrians and train a clique of Chronomancers to defend Yavdlom, fix certains problems and correct TSR plotholes -- which may explain why certain modules like X9-the Savage Coast, X6-Quagmire, X4-Master of the Desert Nomads and X5-the Temple of Death get retconned and placed in new spots on the Mystaran timeline.
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Chronomancy might also explain the divinarchy as a system of government that serves as a group effort to stabilize the timeline, and perhaps Yavdlom's isolationnism in foreign matters, as to avoid butterfly effects in world politics.
(artwork from: https://www.artstation.com/artwork/d8dG3Q)