How can I see the date of the last trade taken?

I'm trying to limit my robot to one buy trade and one sell trade per day. If I've done this right, I have applied a label to my trades:

                    var result = ExecuteMarketOrder(TradeType.Sell, Symbol, Volume, "Sell", StopLoss, TakeProfit);
                    if (!result.IsSuccessful)
                    {
                        Print("Stoping cBot due to failure to execute market order");
                        Stop();
                    }

But when I try to retrieve the date and compare it to the current date:

                HistoricalTrade lastSellTradeDate = History.FindLast("Sell");
                DateTime openTime = MarketSeries.OpenTime.LastValue;
                DateTime currentDate = MarketSeries.OpenTime.LastValue.Date;

                Print("Current Date is " + currentDate + " and last trade was taken " + lastSellTradeDate);

Everything is printed except "lastSellTradeDate". Am I doing something wrong? Or, is there a better way to do this?