Milestone Difference Reporting

As with most projects, we set a milestone date early. Over time milestones can creep out and we update the milestone delivery date.

Today; I can report on a milestone that is overdue. But what I can't report on is what was the first milestone date, and what is the current milestone date, to formulate the difference in movement. (positive or negative).

Which data object is it? Milestone Snapshot contains no records.

Is this achievable? If so, what's your suggestion? If not, is there an alternative (I've read about a custom panel).

TIA