Sounds like it was manually mistyped. Gosh totally assumed it was automated.
If it was mistyped, then it was only on a batch of 'come in early' texts, not on the main system because my phone call confirmed the original date (I didn't tell the call handler the correct date before he checked) and the reminder text was a British-style date.
But the book-earlier text, although from NHS booking, had a different style. For instance, it had my name as well as the booking reference. I even wonderd for a moment or two whether it could be some sort of scam, but it wasn't an email with a link that could be dodgy and although it mentioned a website, it was the official one.
That being said, as I type this, the fact that it mentioned my name does rather suggest that it was individually typed rather than field merge. Even so, I still think there's a good chance it was typed by someone looking at a database. How awful that my default supposition is that there's something wrong with the system when there may not be, just human error.
Normally, this sort of thing just makes me laugh at my own pettiness, it's just the potential for missed appointments that bugged me. There'd be times (including some very zombied patches over the last few months) when I'd have simply thought "Aargh, I wrote the date wrong on my calendar, it's not next week, it's next month!" but not have got round to booking an 'earlier' appointment until after I'd missed my appointment.