Calendar.ISO or really all of elixir’s core datetime handling does indeed not support leap seconds. Calendars are a behaviour though, so in theory you could have another calendar, which handles leap seconds. Practically the calendar alone is likely not enough though and it would need adjustments and considerations in places outside of those calendar implementations as well.
Iirc the latest stance of the elixir team I saw on github discussions was that leap second support would require more input and thought.
