std::chrono::system_clock(3)

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)