cppdraft_translate/cppdraft/time/clock/utc/overview.md

[time.clock.utc.overview]

30 Time library [time]

30.7 Clocks [time.clock]

30.7.3 Class utc_clock [time.clock.utc]

30.7.3.1 Overview [time.clock.utc.overview]

namespace std::chrono {class utc_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<utc_clock>; static constexpr bool is_steady = unspecified; static time_point now(); templatestatic sys_time<common_type_t<Duration, seconds>> to_sys(const utc_time& t); templatestatic utc_time<common_type_t<Duration, seconds>> from_sys(const sys_time& t); };}

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 1:

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 constantsys_days{1972y/1/1} - sys_days{1970y/1/1} (63'072'000s) from the utc_time.

— end note]

[Example 1:

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]

2

#

utc_clock is not a Cpp17TrivialClock unless the implementation can guarantee that utc_clock​::​now() does not propagate an exception.

[Note 2:

noexcept(from_sys(system_clock​::​now())) is false.

— end note]