I went through different posts and Salesforce documentation and don't seem to find any solution which must be somehow possible.
In my custom report type with just one object Activities, I need to access the Created Date of the 'Assigned to' user. In Setup - Report Type - Edit Layout I can find the Created Date of user who created or last modified the activity but there is no such field for the user assigned to the event.
Is there any way I can add it?
vs.
The OwnerId field is the "Assigned To" field. You can see this in the Fields & Relationship section in the Setup screen:
Also see Task object documentation.
UPDATE: Oh, the owner is not available for relationship traversal because it is a polymorphic field - it could be a User or a Queue. You might be able to create a formula field in Task to do this, but most likely you'll have to build automation to populate a custom Date field.
I have figured out the solution. Adding the Created Date of the 'Assigned to' User is actually possible directly in the report type. The date field is hidden under the Role: Report Type -> Edit Layout -> Add fields related via lookup - Assigned to -> Role -> User -> Created Date
