When KST calendars and UTC warehouses disagree
Day-one retention can shift when midnight moves—not when the product does.
Application teams serving Korea often mix UTC event stores with KST product calendars. A cohort day that spans the wrong midnight rewrites every retention cell without a single feature ship.
SDK upgrades that change anonymous-to-known stitching create similar ghosts. Returning users look new; day-seven rates swing for pipeline reasons.
Before celebrating a lift, list SDK versions, identity rules, and timezone settings beside the chart. Our Session Continuity Review always opens with that checklist.