I am trying to understand how the "market on open" and "market on close" order types would work for emini S&P 500 futures. I understand how such an order would operate for ETF purchases (an OPG order is initiated at 9:30 AM EST and a CLO order is initiated at 4:00 PM EST), but since the trading times are different for futures, when would these equivalent order types be placed in eastern standard time if trading index futures like the emini?