The easiest way out of this particular puzzle is, I believe, to say
that two datetimes identical except for `fold` do _not_ compare equal.
`fold` breaks the tie in the obvious way (the one with fold==1 is
"greater").
I am afraid you are right, but proving that we will not break naive (fold unaware) programs will be harder in this case. Let me think some more about this.
Meanwhile, would you see any problem with not(x - y) not implying x == y?