How do I create a custom formula field using date from another formula field in a summary report?

How do I create a custom formula field using date from another formula field in a summary report?

 I've created a couple of custom formula fields in my summary report and I'd like to use these fields to create other fields showing percentages and other formulas. However when I go to the add formula section like I had previously, none of the formula fields that I created in the summarize section are able to be chosen as a column to use in a formula. What's the work around? I've included screen shots of the fields I'm trying to use. Thanks,