KST cohorts and the quiet clock problem
Day-one retention can 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 identity 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 the cohort from raw events.