Stripe assigns cohorts to subscribers based off of when they started generating positive Monthly Recurring Revenue (MRR). In the case of trials, this occurs when the trial has ended and they have converted. Otherwise, this is whenever they start their first paid subscription.
For all subscribers in a cohort, we measure retention by checking to see whether a subscriber churned during a given month, using Coordinated Universal Time (UTC). Retention is the percentage of subscribers from that cohort who haven’t yet churned by the end of the month.
After a subscriber churns, if they re-subscribe, they will be assigned to another cohort and could therefore appear multiple times in the retention chart.