Yes. As I said in my first post create a form. It would need to be in "Single" or "Continuous" view because you can't have a button on a form in "Datasheet" view.
Add the following code to the button's On Click event.
Code:
If IsNull(Me.[Start Time]) Then
Me.[Start Time] = Now()
ElseIf IsNull(Me.[End Time]) Then
Me.[End Time] = Now()
End If
BTW You appear to have spaces in the names of some fields and tables. This is not considered to be "Good Practice" so I would advise that you change names like "Start Time" to "StartTime" or Start_Time".