29 Time library [time]

29.7 Clocks [time.clock]

29.7.2 Class system_clock [time.clock.system]

29.7.2.1 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; }; }
1
#
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]

29.7.2.2 Members [time.clock.system.members]

🔗
using system_clock::rep = unspecified;
1
#
Constraints: system_clock​::​duration​::​min() < system_clock​::​duration​::​zero() is true.
[Note 1: 
This implies that rep is a signed type.
— end note]
🔗
static time_t to_time_t(const time_point& t) noexcept;
2
#
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.
🔗
static time_point from_time_t(time_t t) noexcept;
3
#
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.

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

🔗
template<class charT, class traits, class Duration> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const sys_time<Duration>& tp);
1
#
Constraints: treat_as_floating_point_v<typename Duration​::​rep> is false, and Duration{1} < days{1} is true.
2
#
Effects: Equivalent to: return os << format(os.getloc(), STATICALLY-WIDEN<charT>("{:L%F %T}"), tp);
3
#
[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]
🔗
template<class charT, class traits> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const sys_days& dp);
4
#
Effects: os << year_month_day{dp}.
5
#
Returns: os.
🔗
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);
6
#
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.
7
#
Returns: is.