[tz] Question about leap seconds

[random832 at fastmail.us]

Two things: Can a transition time ever reasonably fall _on_ a leap
second, and can the zic input format represent this case?

I am sketching out a design for a new time zone file format, and it
occurs to me that it might be a good idea to specify transitions with
POSIX timestamps so the data regarding one time zone is independent of
whether leap seconds are enabled or not.