It ought to be obvious by now that POSIX time is a cruel joke not suitable for timing anything. So the question is, what is to be done about it? I suggest the following:
1. The adoption of a standard for real time, something like CLOCK_TAI. (It is too bad that CLOCK_REALTIME is already taken for such a ridiculously non real time clock).
2. The standardization of an interface for smoothed UTC, something like CLOCK_SLS.
3. The standardization of an interface for strict (non-smoothed,non-slewed) UTC, something like CLOCK_UTC.
4. Deprecate CLOCK_REALTIME.
5. At the kernel level, do all timekeeping using TAI or the equivalent. Convert to UTC, UTC-SLS as necessary.