When UNIX is converted into the local culture leap seconds are added (along with time zone, daylight savings time, et al), defining dates before 1970 is irrelevant (and also untrue, UNIX times can be negative), and as systems are moving rapidly to 64 bit the 2038 deadline will be irrelevant (e.g. my Chrome AND their server software are both 64 bit already).

> There are very few good reasons to choose a Unix timestamp to represent a date when compared with ISO-8601.

Complete lack of string parsing is a good one.