private void PostponeButton_Click(object sender, RoutedEventArgs e)
 {
     // get the number of minute
     if (PostponeComboBox.SelectedItem.Equals(OneMinItem))
     {
         numberOfMinute = 1;
     }
     else if (PostponeComboBox.SelectedItem.Equals(FiveMinItem))
     {
         numberOfMinute = 5;
     }
     else if (PostponeComboBox.SelectedItem.Equals(TenMinItem))
     {
         numberOfMinute = 10;
     }
     else if (PostponeComboBox.SelectedItem.Equals(ThirtyMinItem))
     {
         numberOfMinute = 30;
     }
     else if (PostponeComboBox.SelectedItem.Equals(OneHourItem))
     {
         numberOfMinute = 60;
     }
     else if (PostponeComboBox.SelectedItem.Equals(EightHourItem))
     {
         numberOfMinute = 480;
     }
     else if (PostponeComboBox.SelectedItem.Equals(OneDayItem))
     {
         numberOfMinute = 1440;
     }
     commonSet.PostponePrintingTime(numberOfMinute);
     commonSet.SaveNextTime();
     this.Close();
 }