We use Service Management (next gen) heavily. What we see repeatedly is that Resolved (date) is never set.
The status is set to Done. History shows that as well. But
a) the ticket is unresolved
b) there is no "resolution" field available in next gen
c) as a result we have no way of finding the equivalent of Resolved date.
There must be a solution to this, but we sure cannot find it looking around.