private void timetableDataGridView_CellValidating(object sender, DataGridViewCellValidatingEventArgs e)
        {
            string headerText = timetableDataGridView.Columns[e.ColumnIndex].HeaderText;

            if (!headerText.Equals("Date")) return;
            DateTime date;
            if (!DateTime.TryParse(e.FormattedValue.ToString(), out date) && date > System.DateTime.Now)
            {
                timetableDataGridView.Rows[e.RowIndex].ErrorText = "attention";
                e.Cancel = true;
            }
        }

        void timetableDataGridView_CellEndEdit(object sender, DataGridViewCellEventArgs e)
        {
            // Clear the row error in case the user presses ESC.
            timetableDataGridView.Rows[e.RowIndex].ErrorText = String.Empty;
        }