POSIX time isn't quite an elapsed time standard, thanks to the absence of leap seconds.

Wall clock time should never be used to measure time when these seconds matter. Like I said in grandparent comment:

> for time measurements you should always use a monotonic clock, not wall clock.

POSIX time isn't a time elapsed standard. Yes. But neither is iso8601, timestamp+timezone, everyday speak time "tomorrow at noon", or any of these other options.

So I'm not sure what your point is, other than to say that a hammer is not a screwdriver.