std::chrono::floor(std::chrono::time_point)(3)

NAME
std::chrono::floor(std::chrono::time_point) - std::chrono::floor(std::chrono::time_point)

Synopsis
   Defined in header <chrono>
   template <class ToDuration, class Clock, class Duration>
   constexpr std::chrono::time_point<Clock, ToDuration> (since C++17)
   floor(const std::chrono::time_point<Clock, Duration>& tp);
   Returns the largest time point t representable in ToDuration that is smaller or
   equal to tp.
   The function does not participate in the overload resolution unless ToDuration is a
   specialization of std::chrono::duration.

Parameters
   tp - time point to round down

Return value
   tp rounded down to the next time point using duration of type ToDuration.

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 Clock, class FromDuration,
             class = std::enable_if_t<detail::is_duration_v<To>>>
   constexpr std::chrono::time_point<Clock, To>
       floor(const std::chrono::time_point<Clock, FromDuration>& tp)
   {
       return std::chrono::time_point<Clock, To>{
           std::chrono::floor<To>(tp.time_since_epoch())};
   }

Example
// Run this code
 #include <iostream>
 #include <chrono>
 #include <string>
 template <typename TimePoint>
 std::string to_string(const TimePoint& time_point)
 {
     return std::to_string(time_point.time_since_epoch().count());
 }
 int main()
 {
     using namespace std::literals::chrono_literals;
     using Sec = std::chrono::seconds;
     std::cout << "Time point\t" "Cast\t" "Floor\t" "Round\t" "Ceil\n";
     std::cout << "(ms)\t\t" "(s)\t" "(s)\t" "(s)\t" "(s)\n";
     for (const auto value_ms: {5432ms, 5678ms}) {
         std::chrono::time_point<std::chrono::system_clock, std::chrono::milliseconds>
             time_point_ms(value_ms);
         std::cout
             << to_string(time_point_ms) << "\t\t"
             << to_string(std::chrono::time_point_cast<Sec>(time_point_ms)) << "\t"
             << to_string(std::chrono::floor <Sec>(time_point_ms)) << "\t"
             << to_string(std::chrono::round <Sec>(time_point_ms)) << "\t"
             << to_string(std::chrono::ceil <Sec>(time_point_ms)) << "\n";
     }
 }

Output:
 Time point Cast Floor Round Ceil
 (ms) (s) (s) (s) (s)
 5432 5 5 5 6
 5678 5 5 6 6

See also
   time_point_cast converts a time point to another time point on the
   (C++11) same clock, with a different duration
                        (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)
   floor(std::chrono::duration) converts a duration to another, rounding down
   (C++17) (function template)
   floor
   floorf nearest integer not greater than the given value
   floorl (function)
   (C++11)
   (C++11)