private void PerformancesComboBox_SelectionChanged(object sender, SelectionChangedEventArgs e)
        {
            string name = e.AddedItems[0].ToString();

            dateComboBox.Items.Clear();

            foreach (DateTime date in cashbox.GetDateTimes(name))
            {
                dateComboBox.Items.Add(date.ToShortDateString());
            }
        }