std::chrono::floor(std::chrono::duration)

template <class T> struct is_duration : std::false_type {};
template <class Rep, class Period> struct is_duration<
    std::chrono::duration<Rep, Period>> : std::true_type {};
 
template <class To, class Rep, class Period,
          class = std::enable_if_t<is_duration<To>{}>>
constexpr To floor(const duration<Rep, Period>& d)
{
    To t = std::chrono::duration_cast<To>(d);
    if (t > d)
        --t;
    return t;
}