Time stamp with approval date

Hi,

I would like to know if there is a possibility to customize the "time stamp" in the workflow tab for PO, so it can display the date/time of workitem approval ?.see picture 1. 

Actually the time stamp shows only the creation date of workitem. see picture 2.

If not we would like to know where to find this data (date/time of workitem release/approval) and add a new columnto holdthis date.

Thank you in advance.