Leap seconds are their own nightmare. UNIX time ignores them, btw, so that the unix epoch is 86400*number of days since 1/1/1970 + number of seconds since midnight. The behavior at the instance of a leap second is undefined.

That's a good way of describing that. It's far too easy to pretend UNIX timestamps would correspond to a stopwatch counting from 1/1/1970.

Undefined behavior is worse than complicated defined behavior imo.