รบกวนถามเรื่องคำนวณเรื่องเวลาครับ vb.net รบกวนขอคำแนะนำหรือตัวอย่างในการคำนวณเวลาออกมาเป็นชั่วโมง

  Private Sub BtnCheck_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles BtnCheck.Click
        If TxtEmpID.Text = "" Then
            MsgBox("Please input EmpID", MsgBoxStyle.OkOnly)
            TxtEmpID.Text = ""
            TxtEmpName.Text = ""
            TxtEmpID.Focus()
        Else
               Sumrow = DataGridView1.Rows.Count - 1
            For i As Integer = 0 To Sumrow
                Col1 = DataGridView1.Rows(i).Cells(0).Value 'col1 as date
                Col2 = DataGridView1.Rows(i).Cells(1).Value
                Col3 = DataGridView1.Rows(i).Cells(2).Value
                Col4 = DataGridView1.Rows(i).Cells(3).Value
                Col5 = DataGridView1.Rows(i).Cells(4).Value
                Col6 = DataGridView1.Rows(i).Cells(5).Value
                Col7 = DataGridView1.Rows(i).Cells(6).Value
                Col8 = DataGridView1.Rows(i).Cells(7).Value

                SumTimeCol = (Col2 - Col1) + (Col4 - Col3) + (Col6 - Col5) + (Col8 - Col7)  'SumTimeCol  as timespan
                DataGridView1.Rows(i).Cells(8).Value = SumTimeCol.ToString.Substring(0, 5)
                'รวมเวลา Column ที่ 9
                SumTime = SumTime + SumTimeCol             'sumtime as date
            Next
            TxtSumH.Text = SumTime.ToString.Substring(11, 8)
        End If
    End Sub