UTC seconds are the same as SI seconds. Or rather, there is no defined term "UTC second". You may be thinking of seconds-since-epoch counts that don’t include l

UTC seconds are the same as SI seconds. Or rather, there is no defined term "UTC second". You may be thinking of seconds-since-epoch counts that don’t include leap seconds, like in POSIX timestamps. That's not UTC though. UTC is a naming scheme for points in time, not a prescription for how to count seconds. What's true is that the exact number of seconds between future UTC times isn't known, because leap seconds aren't known arbitrarily in advance. But that doesn’t usually affect measurements of time. It does affect adding (large) SI-second counts to the current time to compute a future time. The exact UTC designation of that future time may not be known in advance.