Try this
Code:
Private Sub cmdDisplay_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles cmdDisplay.Click
Dim strDate As String = InputBox("Please enter the date of the service.")
If strDate <> "" Then
Dim service_date As Date = CDate(strDate)
Dim due_date As Date = service_date.AddDays(30)
ListBox1.Items.Add(service_date)
ListBox1.Items.Add(due_date)
End If
End Sub
You may design a form contain DateTimePicker control to let the user choose the date instead of input it via the ugly InputBox.