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; }; }

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]

30.7.2.2 Members [time.clock.system.members]

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using system_clock::rep = unspecified;

Constraints: system_clock::duration::min() < system_clock::duration::zero() is true.

[Note 1:

This implies that rep is a signed type.

— end note]

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static time_t to_time_t(const time_point& t) noexcept;

Returns: A time_t object that represents the same point in time as t when both values are restricted to the coarser of the precisions of time_t and time_point.

It is implementation-defined whether values are rounded or truncated to the required precision.

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static time_point from_time_t(time_t t) noexcept;

Returns: A time_point object that represents the same point in time as t when both values are restricted to the coarser of the precisions of time_t and time_point.

It is implementation-defined whether values are rounded or truncated to the required precision.

30.7.2.3 Non-member functions [time.clock.system.nonmembers]

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template<class charT, class traits, class Duration>
  basic_ostream<charT, traits>&
    operator<<(basic_ostream<charT, traits>& os, const sys_time<Duration>& tp);

Constraints: treat_as_floating_point_v<typename Duration::rep> is false, and Duration{1} < days{1} is true.

Effects: Equivalent to: return os << format(os.getloc(), STATICALLY-WIDEN<charT>("{:L%F %T}"), tp);

[Example 1: cout << sys_seconds{0s} << '\n'; // 1970-01-01 00:00:00 cout << sys_seconds{946'684'800s} << '\n'; // 2000-01-01 00:00:00 cout << sys_seconds{946'688'523s} << '\n'; // 2000-01-01 01:02:03 — end example]

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template<class charT, class traits>
  basic_ostream<charT, traits>&
    operator<<(basic_ostream<charT, traits>& os, const sys_days& dp);

Effects: os << year_month_day{dp}.

Returns: os.

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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,
                sys_time<Duration>& tp, basic_string<charT, traits, Alloc>* abbrev = nullptr,
                minutes* offset = nullptr);

Effects: Attempts to parse the input stream is into the sys_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.