Yes that was a bad example since the Stock Exchange didn't open on 9/11. What I'm trying to point out is I could buy or sell ETF's at any time during the time the stock market is opened. I think you know what I mean.
I knew what you meant, and as I said, it is a pretty unimportant distinction, especially for index funds, and also especially for someone who has a long-term investment strategy for retirement. It is really hard to time the market, when you are looking at which month to buy or sell. It is essentially impossible for a casual investor to time the market to account for the hours difference in buying/selling ETFs or mutual funds.
Being able to sell ETFs any hour the market is open, vs having a mutual fund transaction take effect only at the end of the trading day, is a pretty meaningless reason to choose ETFs over mutual funds.