[time.clock.system]

# 30 Time library [[time]](./#time)

## 30.7 Clocks [[time.clock]](time.clock#system)

### 30.7.2 Class system_clock [time.clock.system]

#### [30.7.2.1](#overview) Overview [[time.clock.system.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_tstatic time_t to_time_t (const time_point& t) noexcept; static time_point from_time_t(time_t t) noexcept; };}

[1](#overview-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L2663)

Objects of type system_clock represent wall clock time from the system-wide
realtime clock[.](#overview-1.sentence-1)

Objects of type sys_time<Duration> measure time since
1970-01-01 00:00:00 UTC excluding leap seconds[.](#overview-1.sentence-2)

This measure is commonly referred to as [*Unix time*](#def:Unix_time "30.7.2.1 Overview [time.clock.system.overview]")[.](#overview-1.sentence-3)

This measure facilitates an efficient mapping betweensys_time and calendar types ([[time.cal]](time.cal "30.8 The civil calendar"))[.](#overview-1.sentence-4)

[*Example [1](#overview-example-1)*:   


sys_seconds{sys_days{1970y/January/1}}.time_since_epoch() is 0s[.](#overview-1.sentence-5)

  


sys_seconds{sys_days{2000y/January/1}}.time_since_epoch() is 946'684'800s,
which is 10'957 * 86'400s[.](#overview-1.sentence-6)

  
 — *end example*]

#### [30.7.2.2](#members) Members [[time.clock.system.members]](time.clock.system.members)

[🔗](#lib:rep,system_clock)

`using system_clock::rep = unspecified;
`

[1](#members-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L2686)

*Constraints*: system_clock​::​duration​::​min() < system_clock​::​duration​::​zero() is true[.](#members-1.sentence-1)

[*Note [1](#members-note-1)*:

This implies that rep is a signed type[.](#members-1.sentence-2)

— *end note*]

[🔗](#lib:to_time_t,system_clock)

`static time_t to_time_t(const time_point& t) noexcept;
`

[2](#members-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L2700)

*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 andtime_point[.](#members-2.sentence-1)

It is implementation-defined
whether values are rounded or truncated to the required precision[.](#members-2.sentence-2)

[🔗](#lib:from_time_t,system_clock)

`static time_point from_time_t(time_t t) noexcept;
`

[3](#members-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L2716)

*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 andtime_point[.](#members-3.sentence-1)

It is implementation-defined
whether values are rounded or truncated to the required precision[.](#members-3.sentence-2)

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

[🔗](#lib:operator%3c%3c,sys_time)

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

[1](#nonmembers-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L2736)

*Constraints*: treat_as_floating_point_v<typename Duration​::​rep> is false, andDuration{1} < days{1} is true[.](#nonmembers-1.sentence-1)

[2](#nonmembers-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L2741)

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

[3](#nonmembers-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L2748)

[*Example [1](#nonmembers-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*]

[🔗](#lib:operator%3c%3c,sys_days)

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

[4](#nonmembers-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L2766)

*Effects*: os << year_month_day{dp}[.](#nonmembers-4.sentence-1)

[5](#nonmembers-5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L2770)

*Returns*: os[.](#nonmembers-5.sentence-1)

[🔗](#lib:from_stream,sys_time)

`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](#nonmembers-6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L2785)

*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]](time.parse "30.13 Parsing")[.](#nonmembers-6.sentence-1)

If the parse fails to decode a valid date,is.setstate(ios_base​::​failbit) is called andtp is not modified[.](#nonmembers-6.sentence-2)

If %Z is used and successfully parsed,
that value will be assigned to *abbrev if abbrev is non-null[.](#nonmembers-6.sentence-3)

If %z (or a modified variant) is used and successfully parsed,
that value will be assigned to *offset if offset is non-null[.](#nonmembers-6.sentence-4)

Additionally, the parsed offset will be subtracted
from the successfully parsed timestamp
prior to assigning that difference to tp[.](#nonmembers-6.sentence-5)

[7](#nonmembers-7)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L2802)

*Returns*: is[.](#nonmembers-7.sentence-1)