Objects of class system_clock represent wall clock time from the system-wide
realtime clock.

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; // Map to C API static time_t to_time_t (const time_point& t) noexcept; static time_point from_time_t(time_t t) noexcept; }; }

```
using system_clock::rep = unspecified;
```

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

```
static time_point from_time_t(time_t t) noexcept;
```

Objects of class steady_clock represent clocks for which values of time_point
never decrease as physical time advances and for which values of time_point advance at
a steady rate relative to real time.

That is, the clock may not be adjusted.

namespace std::chrono { class steady_clock { public: using rep = unspecified; using period = ratio<unspecified, unspecified>; using duration = chrono::duration<rep, period>; using time_point = chrono::time_point<unspecified, duration>; static constexpr bool is_steady = true; static time_point now() noexcept; }; }

Objects of class high_resolution_clock represent clocks with the
shortest tick period.

namespace std::chrono { class high_resolution_clock { public: using rep = unspecified; using period = ratio<unspecified, unspecified>; using duration = chrono::duration<rep, period>; using time_point = chrono::time_point<unspecified, duration>; static constexpr bool is_steady = unspecified; static time_point now() noexcept; }; }