Background
Earlier, one of the biggest limitations was that, once a user was placed into an audience, they could never leave. But that has changed with the new dynamic audience features that were announced at the Firebase Summit in October 2018. With dynamic audiences, users will join an audience when they meet the criteria for it, but also leave that audience when they no longer belong to the audience using a feature called 'Membership Duration'.
Feature
Here's how Membership Duration
works: Your app will continually evaluating whether or not your users belong to each one of these audiences that you've defined, and is "renewing" your user to audiences that they've qualified for. What this duration field will do is look at a user's last "I've qualified for an audience" time and remove them from the audience if that time was longer than the current membership duration.
Solution
In your case, you just need to add a Membership Duration of 30 days to the audience with app_open
event to get the required audience.

Please note that audience populating starts only after a custom audience is created and doesn't use previous data.