27 Time library [time]

27.7 Clocks [time.clock]

27.7.2 Class system_­clock [time.clock.system]

27.7.2.1 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]

27.7.2.2 Members [time.clock.system.members]

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using system_clock::rep = unspecified;
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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;
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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;
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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.

27.7.2.3 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);
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Constraints: treat_­as_­floating_­point_­v<typename Duration​::​rep> is false, and Duration{1} < days{1} is true.
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Effects: Equivalent to: auto const dp = floor<days>(tp); return os << format(os.getloc(), STATICALLY-WIDEN<charT>("{} {}"), year_month_day{dp}, hh_mm_ss{tp-dp});
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[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);
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Effects: os << year_­month_­day{dp}.
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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);
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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.
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Returns: is.