30 Time library [time]

30.7 Clocks [time.clock]

30.7.2 Class system_clock [time.clock.system]

30.7.2.1 Overview [time.clock.system.overview]

namespace std::chrono { class system_clock { public: using rep = see below; using period = ratio<unspecified, unspecified>; using duration = chrono::duration<rep, period>; using time_point = chrono::time_point<system_clock>; static constexpr bool is_steady = unspecified; static time_point now() noexcept; // mapping to/from C type time_t static time_t to_time_t (const time_point& t) noexcept; static time_point from_time_t(time_t t) noexcept; }; }
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Objects of type system_clock represent wall clock time from the system-wide realtime clock.
Objects of type sys_time<Duration> measure time since 1970-01-01 00:00:00 UTC excluding leap seconds.
This measure is commonly referred to as Unix time.
This measure facilitates an efficient mapping between sys_time and calendar types ([time.cal]).
[Example 1: 
sys_seconds{sys_days{1970y/January/1}}.time_since_epoch() is 0s.

sys_seconds{sys_days{2000y/January/1}}.time_since_epoch() is 946'684'800s, which is 10'957 * 86'400s.

— end example]