I'm confused by the many contradictory descriptions I see about how UTC leap seconds are accounted for. I understand that there are various ways to handle them in common practice, and I've seen a variety of formal definitions. But it seems that in scientific practice, they are simply omitted: there is no UTC Julian Day (that is no JD(UTC) value) corresponding to any times during a leap second, and things like ephemeris are generally not reported for leap seconds. There are, of course, events that take place during leap seconds, but if one wants to refer to the time at which they occur, one uses a different timekeeping system (e.g. UT1 or TT).
Is that right? It makes perfect sense as a way to accommodate the ambiguities that leap seconds introduce, and it in fact corresponds to the way some systems (e.g. POSIX) implement them; but it doesn't quite match the definitions I've seen.