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std::chrono::system_clock(3)  C++ Standard Libary 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 ad-
       justed 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  Coordi-
       nated
	  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::format-
       ter that	formats
	  (C++20)				 a  sys_time  according	to the
       provided	format
						(class	template   specializa-
       tion)

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	repre-
       senting 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 adjust-
       ment, otherwise
				   false
				   (public static member constant)

Member functions
	  now	      returns a	std::chrono::time_point	representing the  cur-
       rent 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, how-
       ever, 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	avail-
       able
	  (C++11)		(class)

http://cppreference.com		  2022.07.31	  std::chrono::system_clock(3)

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