Actually the negative leap second is "more monotonic" than the regular one. Normally a leap second adds an extra second (58, 59, 60, 00), but a negative leap second skips one (58, 00).

Uniquely specifying the four times in the first sequence as UNIX timestamps isn't strictly possible, but it is possible with negative leap seconds.

Now this is going to sound crazy but what if we just extend the year by a few seconds by default? Then we can just skip seconds as needed.

phew!