An,
  In this case, you would not need to save the difference between the Start
and Finish.
  Since you have a Start and Finish saved to the table, you would just need
to recalculate the difference "on the fly" in any subsequent Form, Query, or
Report... just the way you did on the original form...
       = Start - Finish

  For example, if you had
      Price * Qty = LineTotal
you would not save the LineTotal, but reclaculate it from Price * Qty
whenever needed.

  As a general rule... don't save a value in a table that can be re-derived
from already saved values.

  Now... if you "must" save the difference, then add a field to your table
called, for example, [TimeDiff].  Place a text control on the form, with a
ControlSource of TimeDiff,  and use the AfterUpdate event of both Start and
Finish to trigger this code...

Private Sub Start_AfterUpdate()
   Me.TimeDiff = Start-Finish
End Sub
                    and...
Private Sub Finish_AfterUpdate()
   Me.TimeDiff = Start-Finish
End Sub

   If either Start or Finish change, the saved TimeDiff will be updated.

Why not make the [period] field the third text box? In the afterupdate event
in the "Finish" field write

Period = [finish] - [start]

Make the "period" field disabled and read only.