Hmm, perhaps this will work for you.
1. Add two Date Picker Content Controls to your document. Give one the title "In" and the other one the title "Out". Format both so that they only show the time (scroll to the bottom of the format options and maybe pick h:mm:ss am/pm).
2. Add a third Plain Text Content Control and give it the title "Delta".
3. Then add the following macro to your ThisDocument module
Code:
Private Sub Document_ContentControlOnExit(ByVal ContentControl As ContentControl, Cancel As Boolean)
Dim tIn As Date, tOut As Date, tDelta As Date
tIn = ActiveDocument.SelectContentControlsByTitle("In")(1).Range.Text
tOut = ActiveDocument.SelectContentControlsByTitle("Out")(1).Range.Text
tDelta = tOut - tIn
ActiveDocument.SelectContentControlsByTitle("Delta")(1).Range.Text = Format(tDelta, "h:mm:ss")
End Sub