30 Time library [time]

30.7 Clocks [time.clock]

30.7.3 Class utc_clock [time.clock.utc]

30.7.3.2 Member functions [time.clock.utc.members]

static time_point now();

Returns: from_sys(system_clock::now()), or a more accurate value of utc_time.

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template<class Duration>
  static sys_time<common_type_t<Duration, seconds>>
    to_sys(const utc_time<Duration>& u);

Returns: A sys_time t, such that from_sys(t) == u if such a mapping exists.

Otherwise u represents a time_point during a positive leap second insertion, the conversion counts that leap second as not inserted, and the last representable value of sys_time prior to the insertion of the leap second is returned.

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template<class Duration>
  static utc_time<common_type_t<Duration, seconds>>
    from_sys(const sys_time<Duration>& t);

Returns: A utc_time u, such that u.time_since_epoch() - t.time_since_epoch() is equal to the sum of leap seconds that were inserted between t and 1970-01-01.

If t is exactly the date of leap second insertion, then the conversion counts that leap second as inserted.

[Example 1: auto t = sys_days{July/1/2015} - 2ns; auto u = utc_clock::from_sys(t); assert(u.time_since_epoch() - t.time_since_epoch() == 25s); t += 1ns; u = utc_clock::from_sys(t); assert(u.time_since_epoch() - t.time_since_epoch() == 25s); t += 1ns; u = utc_clock::from_sys(t); assert(u.time_since_epoch() - t.time_since_epoch() == 26s); t += 1ns; u = utc_clock::from_sys(t); assert(u.time_since_epoch() - t.time_since_epoch() == 26s); — end example]