Systematic strategies output target positions; the market determines fills. Two portfolios with identical signals can diverge if one trades at the close and another at 10:30 AM.
Backtests should document the assumed execution price and stick to it live.
Market on close (MOC)
Market-on-close orders aim to participate in the closing auction at or near the official closing price (subject to exchange rules and liquidity).
Pros:
- Matches end-of-day signals (common for daily data strategies).
- Large liquidity at the close for many equities and ETFs.
- Simple to model in backtests using closing prices.
Cons:
- Exposure to closing auction imbalance and last-minute news.
- Everyone knows the close matters - crowding possible.
Market on open (MOO)
Similar logic at the opening auction. Useful when signals use overnight information and want the first tradable print.
Gap risk between prior close and open is part of the strategy.
Intraday execution
Trading during the session (mid-morning, lunch, last hour) can:
- Reduce close concentration if your broker and algos support it.
- Align with intraday signals (not all systems use daily bars).
- Introduce partial fill and schedule complexity.
Modeling is harder: you need intraday data and realistic assumptions about spreads at each time.
Matching backtest to live
| If your signal uses... | Natural execution anchor |
|---|---|
| Prior close indicators, trade today | Open or early session |
| Same-day close indicators | Close (with one-bar delay discipline) |
| Intraday bars | Specific bar time or VWAP slice |
A classic mistake is computing signals with today's close and pretending you traded at that close without a lag. That inflates results.
Interaction with rebalancing
Rebalancing on a calendar often picks a fixed clock time (for example, 3:50 PM ET vs official close). Document the minute; slippage can vary by slot.
Multiple strategies
Running several algos at the same timestamp can stress connectivity and buying power. Staggering trade times spreads operational risk if your infrastructure allows it.
Summary
Execution timing is part of the strategy definition. Choose close, open, or intraday to match your data and signal, then keep backtest and live behavior aligned. Changing only the clock can change CAGR as much as changing the signal.
