private void btnAddInterval_Click(object sender, EventArgs e)
        {
            var f = new BillingAfterIntervalEditor(new Billing.DL.Dates(1));

            f.ShowDialog();
            if (f.DialogResult != DialogResult.OK)
            {
                return;
            }
            dates.Add(f.Dates);
            RefreshList();
        }
        private void button2_Click(object sender, EventArgs e)
        {
            if (listInterval.SelectedItems.Count == 0)
            {
                return;
            }
            var index = listInterval.SelectedIndex;
            var f     = new BillingAfterIntervalEditor(dates[index]);

            f.ShowDialog();
            if (f.DialogResult != DialogResult.OK)
            {
                return;
            }

            dates[index] = f.Dates;
            RefreshList();
        }