[rnmr]

جرب هذا

كود :

Dim columnIndex As Integer = 0 ' رقم العمود
Dim h, m As Integer
For Each r As DataGridViewRow In Me.DataGridView1.Rows
   If Not IsNothing(r.Cells(columnIndex).Value) Then
       h += r.Cells(columnIndex).Value.ToString.Split(":")(0)
       m += r.Cells(columnIndex).Value.ToString.Split(":")(1)
   End If
Next
If m > 59 Then
   h += Math.Floor(m / 60)
   m = m Mod 60
End If
Dim result As String = String.Format("{0}:{1}", h, m)

Me.TextBox1.Text = result