Firstly, even though the details section is hidden for the multiple periods, it’s still considered on the report and moving to every formula to the right of it.
Hiding the details just makes it so that you don't have to look at them. They are absolutely still included on the report. If you want to exclude certain records (eg, anything prior to Jan 2010), you do that via selection criteria.
The second problem is I cannot insert a summary for that field anyways.
I can't always insert a summary for a formula either, even when I really think I should be able to. But without knowing something about your actual formulas, I can't offer any advice on that front.
Crystal is a pretty powerful report writer, but it isn't exactly intuitive. Given the difficulties you're having, you should really consider taking a class.
