Understanding Active User Cohorts Over Time for Analysis

We identify "active users" as a user who has done a thing in the last 90 days. But using this kind of cohort means if we look back longer than 90 days, the criteria is not relative to the time period but still in the most recent 90 days... For example "how many active users did we have 12 months ago?" we cannot get this because it will look at users 12 months ago, who have completed a mission in the current "last" 90 days. What am I missing to be able to make such a cohort and have it be usable for all time?