Use both fixed and inherited dates in epics to track delays
Currently for an epic, you can use either the fixed date or the inherited date.
This feature would allow you to have both dates turned "on" (for both start and due ends of the epic). This use case of this is tracking delays (or any other deviations of a plan). That is, when you create an epic, you set a target start date and a target due date. (See https://about.gitlab.com/2018/08/23/epics-roadmap/.) But when you actually do the work, the actual start date and the actual end date become different. But using both types, you can show both on an epic, and also visualize both on a roadmap. This lets you know how late/early you are.
On the roadmap view, you would be able to see the same bar with up to four dates, similar to the ideas in this issue (#3563):