It's not super pretty and easy, but this query will give you the bug ID and the timestamp for the most recent time when the bug was changed to RESOLVED. You could adapt this for CLOSED as well, I am sure. If you wanted access to this information from within the Bugzilla user interface, then you would need to modify the code for your Bugzilla installation to expose this information.
select bugs.bug_id, bugs_activity.bug_when as 'Resolved'
from bugs
left join bugs_activity on bugs.bug_id = bugs_activity.bug_id
and bugs_activity.fieldid=9
and bugs_activity.added='RESOLVED'
and bugs_activity.bug_when = (select max(a.bug_when)
from bugs_activity a
where a.bug_id = bugs.bug_id
and a.fieldid=9
and a.added='RESOLVED')