NAME¶

std::chrono::system_clock - std::chrono::system_clock

Synopsis¶

Defined in header <chrono>
class system_clock; (since C++11)

Class std::chrono::system_clock represents the system-wide real time wall clock.

It may not be monotonic: on most systems, the system time can be adjusted at any
moment. It is the only C++ clock that has the ability to map its time points to
C-style time.

std::chrono::system_clock meets the requirements of TrivialClock.

The epoch of system_clock is unspecified, but most implementations use
Unix Time (i.e., time since 00:00:00 Coordinated Universal Time (UTC), (until C++20)
Thursday, 1 January 1970, not counting leap seconds).
system_clock measures Unix Time (i.e., time since 00:00:00 Coordinated
Universal Time (UTC), Thursday, 1 January 1970, not counting leap (since C++20)
seconds).

Time point family

Defined in namespace std::chrono
template<class Duration>
using sys_time = std::chrono::time_point<std::chrono::system_clock, (since C++20)
Duration>;
using sys_seconds = sys_time<std::chrono::seconds>; (since C++20)
using sys_days = sys_time<std::chrono::days>; (since C++20)

operator<<(std::chrono::sys_time) performs stream output on a sys_time
(C++20) (function template)
from_stream(std::chrono::sys_time) parses a sys_time from a stream according to
(C++20) the provided format
(function template)
std::formatter<std::chrono::sys_time> specialization of std::formatter that formats
(C++20) a sys_time according to the provided format
(class template specialization)

Member types¶

Member type Definition
rep signed arithmetic type representing the number of ticks in the clock's
duration
period a std::ratio type representing the tick period of the clock, in seconds
duration std::chrono::duration<rep, period>, capable of representing negative
durations
time_point std::chrono::time_point<std::chrono::system_clock>

Member constants¶

true if the time between ticks is always constant, i.e.
constexpr bool is_steady calls to now() return values that increase monotonically
[static] even in case of some external clock adjustment, otherwise
false
(public static member constant)

Member functions¶

now returns a std::chrono::time_point representing the current point in time
[static] (public static member function)
to_time_t converts a system clock time point to std::time_t
[static] (public static member function)
from_time_t converts std::time_t to a system clock time point
[static] (public static member function)

Notes¶

The system_clock's time value can be internally adjusted at any time by the
operating system, for example due to NTP synchronization or the user changing the
system's clock. Daylight Savings Time and time zone changes, however, do not affect
it since it is based on the UTC time-zone.

See also¶

steady_clock monotonic clock that will never be adjusted
(C++11) (class)
high_resolution_clock the clock with the shortest tick period available
(C++11) (class)