How to get the actual Effective Start Date of the Grade assigned to the employee?

I'd like to fetch the effective start date of the person's Grades. The actual date when the Grade was assigned to that person. Because currently I'm using the following tables: PER_GRADES_F_TL PGF
, PER_GRADES_F PG
, PER_GRADES P but it gives me values when the Grade was created or updated in the system.