KST calendars, UTC warehouses, and drifting retention
Day boundaries move when clocks disagree—even when the product did not.
Product teams in Korea often mix UTC warehouse timestamps with KST planning calendars. A cohort “day” that straddles the wrong midnight shifts every retention cell without a single feature ship.
SDK upgrades that change anonymous-to-known stitching create related 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 the cohort from raw events before telling a product story.
Session Quality Reviews always open with that checklist—because an exciting narrative is useless if the calendar is wrong.