POSIX time_t still ticked, but the interval between the ticks was 0s, rather than 1s or 2s.
It preserves A (exactly 86400 ticks per calendar day) and C (remains in alignment with UTC) by abandoning B (each tick has a fixed duration)
One could also make ticks on such a day last 86400/86401 or 86400/86399 seconds, keeping the tick duration fixed but only for a given day (a relaxed version of B)