I had made a really dumb mistake in the grammar, and was accepting literals of this form:

datetime 1974-03-25 12:30:00+10

and of course this can’t be distinguished from:

datetime 1974-03-25 12:30:00 + 10

I think I actually meant to make the minutes part of the offset required, and indeed the docs suggest that it is required. But I didn’t do it.

Anyway, I’ve changed the grammar so that the minutes part of an offset is optional for the braced syntax but not for the ANSI-inspired syntax.