KST cohorts and the quiet cost of clock drift
Day boundaries move when warehouses stay on UTC and product calendars stay on KST.
Teams serving users in Korea often mix UTC timestamps with KST product calendars. A cohort “day” that spans the wrong midnight shifts every retention cell without a single feature shipping.
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, rebuild from raw events before telling a product story.
Cohort Integrity Reviews always open with that checklist—because exciting narratives are useless when the calendar is wrong.