Retention is cohorted on time of a users first active session and reports the percentage of users that came back to the app on the exact day reported. For example, day 7 retention will show the % of user who came back to the app exactly 7 days after time of first session.
Formula: For the N-day retention that we report on day X
The denominator is the number of users who first used your app on day X-N
The numerator is the number of users who first used your app on day X-N and returned on day X