I resolved this by implementing the code found in another post.  To resolve
it, I created one "CalcLineItemMinutes" function and pass the DataDOMEvent
argument from the OnAfterChange event of each of the two dates to this
function.  

The following is the code from the CalcLineItemMinutes function:

       public void CalcLineItemMinutes(DataDOMEvent e)

       {
           IXMLDOMNode oNode = e.Site.selectSingleNode("..");
           IXMLDOMNode date1Node = oNode.selectSingleNode("my:StartTime");
           IXMLDOMNode date2Node = oNode.selectSingleNode("my:EndTime");

           string s = "";
           if (date1Node != null && date2Node != null)
           {
               try
               {
                   DateTime date1 = DateTime.Parse(date1Node.text); //
Iso8601ToDate(date1Node.text);
                   DateTime date2 = DateTime.Parse(date2Node.text); //
Iso8601ToDate(date2Node.text);
                   TimeSpan ts = date2.Subtract(date1); // dt2 - dt1;
                   s = ts.TotalMinutes.ToString();
               }
               catch (Exception ex)
               {
                   s = ex.ToString();
               }
           }
           IXMLDOMNode diffNode =
oNode.selectSingleNode("my:LineItemMinutes");
           diffNode.text = s;

       }

The code for the "StartTime" and "EndTime" OnAfterChange events is identical
and has the following:

      // The following function handler is created by Microsoft Office
InfoPath. Do not
       // modify the type or number of arguments.
       [InfoPathEventHandler(MatchPath =
"/my:myFields/my:group1/my:DataRow/my:EndTime", EventType =
InfoPathEventType.OnAfterChange)]
       public void EndTime_OnAfterChange(DataDOMEvent e)
       {
           // Write your code here to restore the global state.

           if (e.IsUndoRedo)
           {
               // An undo or redo operation has occurred and the DOM is
read-only.
               return;
           }
           // A field change has occurred and the DOM is writable.
           // Write your code here.
           CalcLineItemMinutes(e);

       }