Automatic update of a cell every time another one is changed

Good morning to everybody,
I need your valuable help in this matter:
In my database, records' default status is "SOSP" (column S) but afterwards it can become "OK", "S.S." or "REC", changing the corresponding cell of column S. Since the status change is crucial, instead of entering manually in column AP the date on which the change occurs (date format dd/mm/yy) it would extremely nice if this operation were automatic. Please note that everyday not only the database is feed with new records but also other amendments are usually done. Anyway my need is that only the change of status is tracked in column AP i.e. only the changes occurred in column S. I assume that a macro triggered by the Book.save event would work.
I attach a sample book and thank you in advance for your support.

Thanks a lot Richard!