Current Date and time per cell

You require a logic test that applies the Now() stamp when the criteria is met - a fresh change to “Closed” in the monitored cell - but when not met leaves the check cell as it was.

Solution:

=if(AND(i9=“Closed”,Cell(“address”,i9)=Cell(“address”)),Now(),j9)

Based on:

Date of Last Modified for a Cell | Microsoft Community Hub

This requires iterative calculation setting to be turned on, details of which can be found in the thread linked above. 

I have assumed that the cell update test { Cell(“address”,i9)=Cell(“address”) } will work for a dynamically updated rather than user updated cell - this needs to be tested.