You can still have a DateTimePicker control on your form.
And if you wish it to be the last thing entered before adding data to the DataGrid, all you have to do is move the code previously in the SelectedIndexChanged event of the combobox into the ValueChanged event of the DateTimePicker.
When that is done, add these lines to the code that adds to the DataGrid.
cell = New DataGridViewTextBoxCell
cell.Value = DateTimePicker1.Value.ToShortDateString
row.Cells.Add(cell)
And also an additional line &= ";" & row.Cells(x).Value.ToString()
to the SaveToFile method.