You can find this information in the mantis_bug_history table by finding the entry where the status becomes "resolved". Some of the graph by time pages do this as I recall.

You can also try to store this data in a separate field:

1. create a custom field "Date_resolved", type Date, edit by admin only, etc.

2. assign this field to a project

3. create function like:
   

   
   
   function set_resolve_date_to_custom_field( $p_bug_id ) {
   
   $resfield = custom_field_get_id_from_name('Date_resolved');
   
   $p_project_id = bug_get_field( $p_bug_id, 'project_id' );
   
   if( custom_field_is_linked($resfield, $p_project_id) ) {
   
   $resdate = db_now();
   
   custom_field_set_value( $resfield, $p_bug_id, $resdate, false );
   
   }
   
   }
   
   

4. Change the core/bug_api.php:
   find function bug_resolve(....)
   add at the end of the function (before "return true;") following:

set_resolve_date_to_custom_field( $p_bug_id );

What's the result:

• Date_resolved will be set to the sysdate when the bug is set to "resolved"
• Date_resolved will NOT be set when the bug is set to "closed"
• Date_resolved will NOT be cleared when the bug is reopened or smth like that
• The filter will be available to search by Date_resolved

Since there are changes in the core (bug_api.php) and it's possible to differ this field and the real history date of resolving, -
This is NOT the good solution, you may use it only if you can't use the mantis_bug_history table (see the thraxisp comment) - for example, if you want to see the resolve date in the View Issues page.