Backtesting past real-time trades

[Killer Bee]

I trade long or short based upon the closing value of the S&P 500. I have been very successful this year making over 120%. However, last week, we were subject to a 25% decline which could have been avoided had the S&P 500 closed in positive territory.

Is there to backtest using previous buy or sells to based upon the percentage gain or loss of the S&P 500 for particular day?