Thanks hozawa for reply. As I said, I am not finding the details in sys_audit and sys_history_line. But I do see the information in each individual record by going to audit history -> calendar view. That's why I was wondering where else it could be stored. 

In our environment we find some assets have their assigned date value blanked out. And we need to run chargeback report based on the assigned date. So, looking for ways to extract this information by approximating assigned_to audit time as assigned date. However, I am not able to find this information in sys_audit / sys_history_line tables. So, not finding a way to query those values. 

Thx,
Sanya