Retention artefacts from timezone and identity drift
Day-one retention can move when clocks and user IDs move—not 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 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 the merge rules.
Before celebrating a retention lift, ask what changed in the pipeline. 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.
Our Retention Cohort Diagnostics engagements always open with that boring checklist—because the exciting product story is useless if the calendar is wrong.