Hello and welcome!
Here are a few documents that might be of use to you:
To answer your questions more specifically:
the date an incident is designated resolved - look for the im_incident_resolution table. This will have a datetime field im_creation_date and links to the pm_guid via im_incident_guid.
the date an incident is designated as closed - similar advice for this but the table is called im_incident_closure.
the date an incident is designated unresolved - similar advice for this but the table is called im_incident_reopen.
The data is stored as separate records in these tables because you could resolve, close and reopen the same incident a number of times and each time will be datetimestamped.
On behalf of myself and my department head:
Pleased I was able to help