std::chrono::round(std::chrono::duration)(3)

NAME
std::chrono::round(std::chrono::duration) - std::chrono::round(std::chrono::duration)

Synopsis
   Defined in header <chrono>
   template <class ToDuration, class Rep, class Period>
   constexpr ToDuration round( const std::chrono::duration<Rep, Period>& (since C++17)
   d );
   Returns the value t representable in ToDuration that is the closest to d. If there
   are two such values, returns the even value (that is, the value t such that t % 2 ==
   0).
   The function does not participate in the overload resolution unless ToDuration is a
   specialization of std::chrono::duration and
   std::chrono::treat_as_floating_point_v<typename ToDuration::rep> is false

Parameters
   d - duration to convert

Return value
   d rounded to the nearest duration of type ToDuration, rounding to even in halfway
   cases.

Possible implementation
   namespace detail {
   template<class> inline constexpr bool is_duration_v = false;
   template<class Rep, class Period> inline constexpr bool is_duration_v<
       std::chrono::duration<Rep, Period>> = true;
   }
   template <class To, class Rep, class Period,
             class = std::enable_if_t<detail::is_duration_v<To> &&
                    !std::chrono::treat_as_floating_point_v<typename To::rep>>>
   constexpr To round(const std::chrono::duration<Rep, Period>& d)
   {
       To t0 = std::chrono::floor<To>(d);
       To t1 = t0 + To{1};
       auto diff0 = d - t0;
       auto diff1 = t1 - d;
       if (diff0 == diff1) {
           if (t0.count() & 1)
               return t1;
           return t0;
       } else if (diff0 < diff1) {
           return t0;
       }
       return t1;
   }

Example
// Run this code
 #include <iostream>
 #include <iomanip>
 #include <chrono>
 int main()
 {
     using namespace std::chrono_literals;
     using Sec = std::chrono::seconds;
     for (std::cout << "Duration\tFloor\tRound\tCeil\n"
                       "(ms)\t\t(sec)\t(sec)\t(sec)\n";
         auto const d: {
             +4999ms, +5000ms, +5001ms, +5499ms, +5500ms, +5999ms,
             -4999ms, -5000ms, -5001ms, -5499ms, -5500ms, -5999ms, }) {
         std::cout << std::showpos << d.count() << "\t\t"
                   << std::chrono::floor<Sec>(d).count() << '\t'
                   << std::chrono::round<Sec>(d).count() << '\t'
                   << std::chrono::ceil <Sec>(d).count() << '\n';
     }
 }

Output:
 Duration Floor Round Ceil
 (ms) (sec) (sec) (sec)
 +4999 +4 +5 +5
 +5000 +5 +5 +5
 +5001 +5 +5 +6
 +5499 +5 +5 +6
 +5500 +5 +6 +6
 +5999 +5 +6 +6
 -4999 -5 -5 -4
 -5000 -5 -5 -5
 -5001 -6 -5 -5
 -5499 -6 -5 -5
 -5500 -6 -6 -5
 -5999 -6 -6 -5

See also
   duration_cast converts a duration to another, with a different tick
   (C++11) interval
                        (function template)
   floor(std::chrono::duration) converts a duration to another, rounding down
   (C++17) (function template)
   ceil(std::chrono::duration) converts a duration 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)
   round
   roundf
   roundl
   lround
   lroundf
   lroundl
   llround
   llroundf nearest integer, rounding away from zero in halfway
   llroundl cases
   (C++11) (function)
   (C++11)
   (C++11)
   (C++11)
   (C++11)
   (C++11)
   (C++11)
   (C++11)
   (C++11)