Datagridview Column1 - Column2 = Comlum3 ของแต่ละแถว ทำยังไงครับ

foreach(DataGridViewRow dgvr in DataGridView.Rows)
{
Datetime d1 = Convert.ToDateTime(dgvr.Cells[0].Value);
Datetime d2 = Convert.ToDateTime(dgvr.Cells[1].Value);

dgvr.Cells[2].Value = d1.Subtract(d2).ToString();
}

Private Sub DataGridView1_CellValidated(ByVal sender As Object, ByVal e As System.Windows.Forms.DataGridViewCellEventArgs) Handles DataGridView1.CellValidated
        Dim sam = DataGridView1.Rows(1)
        If e.RowIndex > -1 Then
            For Each r As DataGridViewRow In Me.DataGridView1.Rows
                If Not IsDBNull(r.Cells(6).Value) AndAlso Not IsDBNull(r.Cells(5).Value) Then
                    Dim sum = Convert.ToDateTime(r.Cells(6).Value) - Convert.ToDateTime(r.Cells(5).Value)
                    r.Cells(7).Value = sum 
'cell6 timeout cel5 timenow cell7 totaltime
                Else

                End If
            Next
        End If
    End Sub

 Private Sub Button6_Click_1(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button6.Click
        Dim total As TimeSpan
        For r As Integer = 0 To DataGridView1.RowCount - 1
            total = total + DataGridView1.Rows(r).Cells(7).Value
            Label5.Text = total.ToString
        Next
    End Sub