ACH transacations can be transmitted at any point. In the Federal Reserve cycle, there are 4 distributions per day (all times eastern - 12:00 noon, 4:00 pm, 10:00 pm, and 1:00 am). In reality, the distributions generally happen before this time. So, if a bank originates an item at 10:00 AM and delivers it to the Federal Reserve, your bank could receive this item at the 12:00 noon distribution - just 2 hours later. However, with the exception of return items before 12:00 noon, the Fed does not allow for same day posting of entries, so your account should not be debited/credited until the next day, what the Fed calls the settlement date (this is different that an effective date, which is the day the originator says the payment should be made). Only the Fed can set the settlement date.
I don't know about items sent via the private ACH exchange and how they work.
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WC done for US - let's hope we fare better in the Tour de France