[time.clock.system.overview]

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

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

### 30.7.2 Class system_clock [[time.clock.system]](time.clock.system#overview)

#### 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_tstatic time_t to_time_t (const time_point& t) noexcept; static time_point from_time_t(time_t t) noexcept; };}

[1](#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[.](#1.sentence-1)

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

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

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

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


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

  


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

  
 â *end example*]
