Proposed resolution:
This wording is relative to N4835.
Modify [time.clock.utc.overview] as indicated:
-1- In contrast to sys_time, which does not take leap seconds into account, utc_clock and its associated time_point, utc_time, count time, including leap seconds, since 1970-01-01 00:00:00 UTC. [Note: The UTC time standard began on 1972-01-01 00:00:10 TAI. To measure time since this epoch instead, one can add/subtract the constant sys_days{1972y/1/1} - sys_days{1970y/1/1} (63'072'000s) from the utc_time — end note] [Example: clock_cast<utc_clock>(sys_seconds{sys_days{1970y/January/1}}).time_since_epoch() is 0s. clock_cast<utc_clock>(sys_seconds{sys_days{2000y/January/1}}).time_since_epoch() is 946'684'822s, which is 10'957 * 86'400s + 22s. — end example]