Assign a last history of the cell into a column

Hello,

I would need to track a lead time of request approval. I keep those approval status from several approvers of 2-4 people in the cells.

I understand that there is a activity log report. however it still needs manual mapping to identify each approver timestamps and calculate time lag. My goal is assign a last activity timestamp from a certain cell and assign to a dedicated column in the Sheet like the "Modified Date". the reason I can't use "Modified Date" is, it tracks the changes coming from the entire row instead of a certain column. 

Hope I described the situation clearly. I have a business license. 

Thank you in advance.

Hana