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< cpp | chrono | system clock C++ Compiler support Freestanding and hosted Language Standard library Standard library headers Named requirements Feature test macros (C++20) Language support library Concepts library (C++20) Diagnostics library Memory management library Metaprogramming library (C++11) General utilities library Containers library Iterators library Ranges library (C++20) Algorithms library Strings library Text processing library Numerics library Date and time library Input/output library Filesystem library (C++17) Concurrency support library (C++11) Execution control library (C++26) Technical specifications Symbols index External libraries [edit] Date and time library Time point time_point(C++11)
clock_time_conversion(C++20)
(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)
Time of day is_amis_pm(C++20)(C++20)
make12make24(C++20)(C++20)
hh_mm_ss(C++20)
Calendar day(C++20)
month(C++20)
year(C++20)
weekday(C++20)
operator/(C++20)
year_month_day(C++20)
year_month_day_last(C++20)
year_month_weekday(C++20)
year_month_weekday_last(C++20)
(C++20)
weekday_last(C++20)
month_day(C++20)
month_day_last(C++20)
month_weekday(C++20)
month_weekday_last(C++20)
year_month(C++20)
last_speclast(C++20)(C++20)
chrono I/O parse
(C++20)
(C++20)
tzdb_list(C++20)
get_tzdbget_tzdb_listreload_tzdbremote_version(C++20)(C++20)(C++20)(C++20)
sys_info(C++20)
local_info(C++20)
nonexistent_local_time(C++20)
ambiguous_local_time(C++20)
locate_zone(C++20)
current_zone(C++20)
time_zone(C++20)
choose(C++20)
zoned_traits(C++20)
zoned_time(C++20)
time_zone_link(C++20)
leap_second(C++20)
leap_second_info(C++20)
get_leap_second_info(C++20)
C-style date and time [edit] std::chrono::system_clock Member functions system_clock::now system_clock::to_time_tsystem_clock::from_time_t
Time point I/O operator<< formatter<std::chrono::sys_time> from_stream [edit] static std::chrono::system_clock::time_point from_time_t( std::time_t t ) noexcept; (since C++11)Converts t to a time point type, using the coarser precision of the two types.
If time_point has lower precision, it is implementation defined whether the value is rounded or truncated.
A value of type std::chrono::system_clock::time_point representing t.
Run this code
#include <chrono>
#include <ctime>
#include <iostream>
#include <thread>
int main()
{
using namespace std::chrono_literals;
const std::time_t t = std::time(nullptr); // usually has "1 second" precision
const auto from = std::chrono::system_clock::from_time_t(t);
std::this_thread::sleep_for(500ms);
const auto diff = std::chrono::system_clock::now() - from;
std::cout << diff << " ("
<< std::chrono::round<std::chrono::milliseconds>(diff)
<< ")\n";
}
Possible output:
987654321ns (987ms)[edit] See also to_time_t
[static]
converts a system clock time point to std::time_tRetrieved from "
https://en.cppreference.com/mwiki/index.php?title=cpp/chrono/system_clock/from_time_t&oldid=157201"
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