[wahern]

More specifically, POSIX explicitly defines a day as having precisely 86400 seconds. This makes calendar arithmetic trivial, including into the future, which would otherwise be impossible as leap seconds aren't predictable. Any program with a leap day bug is almost certainly not making use of Unix timestamps.

The corollary is that for the most commonly used APIs a Unix second isn't the same thing as an SI second. A Unix second is effectively defined in terms of the civil calendar, not as a fixed quantum of physical time.

Technically this doesn't preclude Unix date-time strings from displaying a 60th second. (And maybe some do.) But it would require unnecessary extra work, introduce inconsistencies (i.e. a generated string for a future date-time that happened to be a leap second would show :59 today but :60 at the moment of the leap second), and invite bugs.