Problem: I would like to filter a View based on a date custom field. Only results between today and 90 days from now should appear.
Solution: Do not use conditional HTML inside the results of a View if you intend to use pagination. Instead, the preferred method is to apply a Query Filter. The settings for the date range filter you described are "UNSIGNED", "between", "TODAY", and "FUTURE_DAY 90"
Problem: I would like to set up a "Bookmark" button so that Users can bookmark posts for reference later. Then I would like to set up a View to show each User their bookmarked posts.
Solution: There is not currently a built-in way to associate Users and Posts, so you would have to create something like this with CRED. You could create a repeating field on the post that stores User IDs, and use CRED to add or remove the current User's ID from this repeating field group on each post.