[pclmulqdq]

> Sending orders at "09:29:59.9999971 at the hope your order arrives at 100ns past 9.30am." What?

This literally does happen, though. One of the things the hyperscalers have convinced the world is that precise time is hard. Precise time is easy if you are willing to pay extra for your hardware. Sub-10-ns precision is unremarkable when you use PTP.

[minimax]

It doesn't happen. All the exchanges have a "Day" order type that you can send before 9:30 that will be live on the book when it opens at 9:30 (or transitions to the "core" session at 9:30, most US exchanges have a premarket session prior to that). The idea of having some sophisticated strategy that sends 100ns before 9:30 is nonsense.

[pclmulqdq]

As far as I know, you're correct that this exact trade probably doesn't happen on the US exchanges - day orders do have a matching phase before market open, so it may be advantageous to slide in right afterward, but you likely wouldn't do it without knowledge of the state of the opening auction.

However, sending things just a hair early for scheduled events to catch an exact time is a pretty well-known trick at this point. I remember complaining to the exchange that their clocks weren't precise enough for this to be reliable.