Subtracting Datetime In Stored Procedure And Displayed In Hh:mm:ss Format

 Hi there,
I am trying to write a stored procedure in which I will retrieve SessionStartDate, SessionEndDate, and Duration (where Duration is calculdated by subtracting SessionEndDate from SessionStartDate).
I was duration in the format  of hours:minutes:seconds.
The stored procedure is pasted below. I am getting the following error.
Syntax error converting datetime from character string. 
 
Any ideas?
 
==============================
 CREATE    PROCEDURE sp_ActiveSessions_UsersBrowsingDurationByDate_List  (
    @websiteID AS int = 0,
    @SelectedDateFrom AS dateTime,
    @SelectedDateTo AS dateTime
) AS

IF DateDiff(d,@SelectedDateTo,@SelectedDateFrom)=0
begin
    set @SelectedDateFrom=null
end

IF ISNULL(@SelectedDateTo, '') = ''
begin
    SET @SelectedDateTo = @SelectedDateFrom
end
SET @SelectedDateTo = DATEADD(d, 1, @SelectedDateTo)

SELECT UserID As 'User ID', SessionStartDate As 'Session Start Date', SessionEndDate AS 'Session End Date', ExitPageTitle As 'Exit Page Title', NumberOfPagesVisited As 'Number of Pages Visited', Convert(datetime, (CONVERT(DATETIME, SessionEndDate, 24) - CONVERT(DATETIME, SessionStartDate, 24)), 101) As 'Duration' FROM ActiveSessions

WHERE UserID != 'Anonymous'
GROUP BY SessionID, UserID, SessionStartDate, SessionEndDate, NumberOfPagesVisited, ExitPageTitle
HAVING (min(SessionStartDate) BETWEEN @SelectedDateFrom AND @SelectedDateTo AND min(SessionEndDate) BETWEEN @SelectedDateFrom AND @SelectedDateTo)
GO


==============================
  <Columns>
                                        <asp:BoundColumn DataField="User ID" HeaderText="User ID"></asp:BoundColumn>
                                        <asp:BoundColumn DataField="Session Start Date" HeaderText="Session Start Date"></asp:BoundColumn>
                                        <asp:BoundColumn DataField="Session End Date" HeaderText="Session End Date"></asp:BoundColumn>
                                        <asp:BoundColumn DataField="Duration" HeaderText="Duration"></asp:BoundColumn>
                                        <asp:BoundColumn DataField="Number Of Pages Visited" HeaderText="Number Of Pages Visited"></asp:BoundColumn>
                                        <asp:BoundColumn DataField="Exit Page Title" HeaderText="Exit Page Title"></asp:BoundColumn>
                                    </Columns>
 
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