29 Time library [time]

29.7 Clocks [time.clock]

29.7.4 Class tai_clock [time.clock.tai] Overview [time.clock.tai.overview]

namespace std::chrono { class tai_clock { public: using rep = a signed arithmetic type; using period = ratio<unspecified, unspecified>; using duration = chrono::duration<rep, period>; using time_point = chrono::time_point<tai_clock>; static constexpr bool is_steady = unspecified; static time_point now(); template<class Duration> static utc_time<common_type_t<Duration, seconds>> to_utc(const tai_time<Duration>&) noexcept; template<class Duration> static tai_time<common_type_t<Duration, seconds>> from_utc(const utc_time<Duration>&) noexcept; }; }
The clock tai_clock measures seconds since 1958-01-01 00:00:00 and is offset 10s ahead of UTC at this date.
That is, 1958-01-01 00:00:00 TAI is equivalent to 1957-12-31 23:59:50 UTC.
Leap seconds are not inserted into TAI.
Therefore every time a leap second is inserted into UTC, UTC shifts another second with respect to TAI.
For example by 2000-01-01 there had been 22 positive and 0 negative leap seconds inserted so 2000-01-01 00:00:00 UTC is equivalent to 2000-01-01 00:00:32 TAI (22s plus the initial 10s offset).
tai_clock is not a Cpp17TrivialClock unless the implementation can guarantee that tai_clock​::​now() does not propagate an exception.
[Note 1: 
noexcept(from_utc(utc_clock​::​now())) is false.
— end note]