To convert not null datetime into null. c#

I would like to make my not null DateTime column null when I do the sum of the entire table so that I do not get any value for date time 1/1/0001 12:00:00. I would like to add whatever possible statement into this method. I tried to do parse it but it doesn’t work, but for Amount it’s perfectly fine.

private void SumOfRecords_Button(object sender, RoutedEventArgs e)
{
    ObservableCollection<Receipt> receipts = new ReceiptDAO().GetAllReceiptsFromPiName();
    Receipt receipt = new Receipt();

    if (receipt.DateTime != null)
    {
        Receipt receipt0 = new Receipt()
        {
            DateTime = DateTime.TryParse(),
            Amount = new ReceiptDAO().SumOfRecords("Amount")
        };

        receipts.Add(receipt);
        this.Dispatcher.Invoke(() => ReceiptList.ItemsSource = receipts);
    }
}

This is the method for SumOfRecords where I am writing my query.

public double SumOfRecords(string columnName)
{
    ObservableCollection<Receipt> receipts = new ReceiptDAO().GetAllReceiptsFromPiName();
    Receipt receipt = new Receipt();
    string commandstring;
    commandstring = "select Sum(" + columnName + ") from " + getTable();
    
    using (SQLiteConnection connection = ConnectToDatabase())
    {
        using (SQLiteCommand command = new SQLiteCommand(commandstring, connection))
        {
            using (SQLiteDataReader reader = command.ExecuteReader())
            {
                reader.Read();
                return reader.GetDouble(0);
            }
        }
    }
}

var where = " WHERE Table.Date <> '0001/01/01 00:00:00'";
commandstring = "select Sum(" + columnName + ") from " + getTable() + where;