Hi Nick,
In order to filter your list “before 90 days” you will need to use a Shared Result which can be generated using the OnScreenLoad event. When customizing your screen, click on “Advanced…” in the top left. Proceed to step 2 and continue to create an Action Set to get the current date, add 90 days to the current date, and then store the result in a Shared Result. It should look something like this
You can now filter by date less than the Shared Result which is the current date + 90 days. Take a look at the “CRM_ActivitiesList” screen if you would like an example as it does similar filtering with dates.
-Audisho