std::chrono::time_point_cast(3)

NAME
std::chrono::time_point_cast - std::chrono::time_point_cast

Synopsis
   Defined in header <chrono>
   template< class ToDuration, class Clock, class Duration >
   std::chrono::time_point<Clock, ToDuration> (since C++11)
                        (until C++14)
       time_point_cast( const std::chrono::time_point<Clock, Duration>
   &t );
   template< class ToDuration, class Clock, class Duration >
   constexpr std::chrono::time_point<Clock, ToDuration> (since C++14)
       time_point_cast( const std::chrono::time_point<Clock, Duration>
   &t );
   Converts a std::chrono::time_point from one duration to another.
   time_point_cast participates in overload resolution only if ToDuration is a
   specialization of std::chrono::duration.

Parameters
   t - time_point to convert from

Return value
   std::chrono::time_point<Clock, ToDuration>(
       std::chrono::duration_cast<ToDuration>(t.time_since_epoch())).

Example
// Run this code
 #include <chrono>
 #include <iostream>
 using namespace std::chrono_literals;
 using Clock = std::chrono::high_resolution_clock;
 using Ms = std::chrono::milliseconds;
 using Sec = std::chrono::seconds;
 template<class Duration>
 using TimePoint = std::chrono::time_point<Clock, Duration>;
 inline void print_ms(const TimePoint<Ms>& time_point)
 {
     std::cout << time_point.time_since_epoch().count() << " ms\n";
 }
 int main()
 {
     TimePoint<Sec> time_point_sec{4s};
     // implicit cast, no precision loss
     TimePoint<Ms> time_point_ms{time_point_sec};
     print_ms(time_point_ms); // 4000 ms
     time_point_ms = TimePoint<Ms>{5756ms};
     print_ms(time_point_ms); // 5756 ms
     // explicit cast, need when precision loss may happen
     // 5756 truncated to 5000
     time_point_sec = std::chrono::time_point_cast<Sec>(time_point_ms);
     print_ms(time_point_sec); // 5000 ms
 }

Output:
 4000 ms
 5756 ms
 5000 ms

See also
   floor(std::chrono::time_point) converts a time_point to another, rounding down
   (C++17) (function template)
   ceil(std::chrono::time_point) converts a time_point to another, rounding up
   (C++17) (function template)
   round(std::chrono::time_point) converts a time_point to another, rounding to
   (C++17) nearest, ties to even
                        (function template)
   duration_cast converts a duration to another, with a different tick
   (C++11) interval
                        (function template)