std::chrono::system_clock

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Date and time library

Time point
time_point (C++11)
clock_time_conversion (C++20)
clock_cast (C++20)
Duration
duration (C++11)
Clocks
system_clock (C++11)
steady_clock (C++11)
is_clock (C++20)
utc_clock (C++20)
tai_clock (C++20)
high_resolution_clock (C++11)
gps_clock (C++20)
file_clock (C++20)
local_t (C++20)
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Calendar
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last_speclast (C++20)(C++20)
`chrono` I/O
parse (C++20)

Time zone
tzdb (C++20)
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sys_info (C++20)
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leap_second_info (C++20)
get_leap_second_info (C++20)
C-style date and time

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std::chrono::system_clock

Member functions
system_clock::now
system_clock::to_time_t
system_clock::from_time_t
Time point I/O
operator<<
formatter<std::chrono::sys_time>
from_stream

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

Classstd::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 ofTrivialClock.

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

Time point family

Defined in namespace`std::chrono`
template<class Duration> using sys_time=std::chrono::time_point<std::chrono::system_clock, Duration>;		(since C++20)
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) (C++20)	performs stream output on a`sys_time` (function template)[edit]
from_stream(std::chrono::sys_time) (C++20)	parses a`sys_time` from a stream according to the provided format (function template)[edit]
std::formatter<std::chrono::sys_time> (C++20)	formatting support for`sys_time` (class template specialization)[edit]

[edit]Member types

Member type	Definition
`rep`	signed arithmetic type representing the number of ticks in the clock's duration
`period`	astd::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>

[edit]Member constants

constexpr bool is_steady

[static]

true if the time between ticks is always constant, i.e. calls tonow() return values that increase monotonically even in case of some external clock adjustment, otherwisefalse
(public static member constant)

[edit]Member functions

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

[edit]Notes

Thesystem_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 Saving Time and time zone changes, however, do not affect it since it is based on theUTC time-zone.

[edit]See also

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