KST midnight and the cohort ghosts it creates
Day boundaries move when warehouses stay on UTC and product calendars stay on KST.
Teams serving users in Korea often mix UTC warehouse timestamps with KST product calendars. A cohort day that spans the wrong midnight shifts every retention cell without anyone shipping a feature.
SDK upgrades that change anonymous-to-known stitching create similar ghosts: returning users appear as new cohorts, and day-seven rates look healthier or worse depending on merge rules.
Before celebrating a retention lift, list SDK versions, identity rules, and timezone settings beside the chart. If those moved, treat the chart as suspect until you rebuild from raw events.
Session Continuity Studies at serverroute always open with that checklist—because the exciting product story is useless if the calendar is wrong.