Analyze features that ship later (or sooner) than initially planned

We would like to analyze features that are shipped later (or sooner) than initially planned.

We already use the existing Release field. I created a custom field to track Initial Release, and I wanted to pick from a list of defined releases, like we do for Release. I could then create a list report with a Calculated field that compares Release and Initial Release dates. When Release date is > Initial Release date, the feature would be "Delayed."

Unfortunately, when trying to create the Calculated field, I couldn't use my custom Initial Release field.

Support explained that "custom relationship fields are not available in the equation builder" (and asked me to add this idea).

Support helped me find a work-around, but it would have been simpler if I could use my original Initial Release field.