27 Time library [time]

27.7 Clocks [time.clock]

27.7.4 Class tai_­clock [time.clock.tai]

27.7.4.1 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; }; }
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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).
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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]

27.7.4.2 Member functions [time.clock.tai.members]

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static time_point now();
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Returns: from_­utc(utc_­clock​::​now()), or a more accurate value of tai_­time.
🔗
template<class Duration> static utc_time<common_type_t<Duration, seconds>> to_utc(const tai_time<Duration>& t) noexcept;
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Returns: utc_time<common_type_t<Duration, seconds>>{t.time_since_epoch()} - 378691210s
[Note 1: 378691210s == sys_days{1970y/January/1} - sys_days{1958y/January/1} + 10s — end note]
🔗
template<class Duration> static tai_time<common_type_t<Duration, seconds>> from_utc(const utc_time<Duration>& t) noexcept;
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Returns: tai_time<common_type_t<Duration, seconds>>{t.time_since_epoch()} + 378691210s
[Note 2: 378691210s == sys_days{1970y/January/1} - sys_days{1958y/January/1} + 10s — end note]

27.7.4.3 Non-member functions [time.clock.tai.nonmembers]

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template<class charT, class traits, class Duration> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const tai_time<Duration>& t);
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Effects: Equivalent to: return os << format(STATICALLY-WIDEN<charT>("{:%F %T}"), t);
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[Example 1: auto st = sys_days{2000y/January/1}; auto tt = clock_cast<tai_clock>(st); cout << format("{0:%F %T %Z} == {1:%F %T %Z}\n", st, tt);
Produces this output: 
2000-01-01 00:00:00 UTC == 2000-01-01 00:00:32 TAI
— end example]
🔗
template<class charT, class traits, class Duration, class Alloc = allocator<charT>> basic_istream<charT, traits>& from_stream(basic_istream<charT, traits>& is, const charT* fmt, tai_time<Duration>& tp, basic_string<charT, traits, Alloc>* abbrev = nullptr, minutes* offset = nullptr);
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Effects: Attempts to parse the input stream is into the tai_­time tp using the format flags given in the NTCTS fmt as specified in [time.parse].
If the parse fails to decode a valid date, is.setstate(ios_­base​::​failbit) is called and tp is not modified.
If %Z is used and successfully parsed, that value will be assigned to *abbrev if abbrev is non-null.
If %z (or a modified variant) is used and successfully parsed, that value will be assigned to *offset if offset is non-null.
Additionally, the parsed offset will be subtracted from the successfully parsed timestamp prior to assigning that difference to tp.
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Returns: is.