Could you please explain this complexity? ELI5 even?

You never settle the trades? Dividends etc?

Hmm maybe I am 5...

Edit: ok yep so I guess what if they repeated the analysis leaving some time around open/close for chance of trades to settle, would the effect disappear or would this chance beef the key factor for the reported gain?

Settlement is when you actually own the stock you bought and the seller actually gets your money. That happens a couple of days after the trade. In the meantime it’s just “as if” but not quite and that has practical consequences. For example you can’t take your money out of the brokerage account until it’s really there.

Thanks! I was figuring this out just now as well thanks.

The analysis is flawed if it doesn't include this as the gains they describe seem essentially unrealisable.

So is the real issue that maybe someone has immediate settlement when the rest don't?

That would be an issue only a few days per year - and not at all for stocks not paying dividends.

All the trades would settle, but you would need extra cash or a margin account. In a cash account, there's rules about selling stocks before the purchase funds have settled.

The move to T+2 settlement last decade reduces the requirements, and T+1 later this decade would reduce them further.

Dividends and corporate actions aren't a big deal. Those all have announcement dates and record dates. If you hold the shares at the close of market on the record date, you will get the dividend or other proceeds. So if you always manage to buy at close, you will always get those benefits. The only complication would be if you want to oppose a merger and have standing to sue; or I guess if you were an injured party in any other shareholder lawsuit. Lots of tax paperwork too.

Thanks for the precision. That makes sense but I wasn't sure.