I’m quite surprised that Unix time is so common. My anecdotal experience with dozens of public and internal APIs over 20 years of development is that the most common format is local time followed by ISO. I actually have almost never seen Unix time.
Programmers are lazy. I've seen more than my fair share of the default behavior of a language like Java implemented. Which usually means just making a system call for the current time and will return the time in whatever time zone the system happens to be set for.
