I am not aware of any operating system kernel that schedules tasks or timer expiration using a strict definition of UTC. Unix time is the most common, and local monotonic (non-decreasing) clocks are becoming common for that as well.

Of course there are application level schedulers that use wall clock time in UTC format, and if you set the clock back your task may run twice, which is a more serious real world problem than running twice around a leap second. That is why no one who is not attached to civil time should use UTC or Unix time for short duration timer expiration in particular.