OTBI Report - Effective Start Date Union

I am creating an OTBI report to extract Union member, Union of the employees and the date from which the employee joined the union. I am using the following subject areas:

- Worker Management - Worker Assignment Real Time

- Workforce Management - Person Real Time

- Workforce Management - Worker Assignment Event Real Time

but, I am not able to find the date from which the employee joined the union. If I put a start date in the report, I see the effective start date of the last change assignment, but it is not the change assignment action done to add the union in the Employee's Profile.