KST calendars and ghost cohorts
Day boundaries move when clocks and identity rules move—not only when the product does.
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 a 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 session or retention lift, list SDK versions, identity rules, and timezone settings beside the chart. Session Quality Diagnostics always opens with that checklist.