Cannot Make Custom Field Formulas with Offered Fields

Raising another flag here because we cannot create custom fields, with some fields that are offered!

I want to be able to quickly calculate - how far my off our current start dates/due dates are from baseline start and due dates. Yes I could great a custom field where I enter in the baseline start date, and then another custom field to run that calculation, but it would be nice if I could just do this all in one spot