In the future, keep your subjet line short, and post the majority of your
question in the body.
SUM is used to add a range of cells together. If you want the time
difference between two cells, simply subtract them, and format cells as
h:mm:ss
=A3-A2
or
=TEXT(A3-A2,"h:mm:ss")
Where A3 is the later time, A2 is the first/earlier time.