< cpp‎ | chrono‎ | duration
Defined in header <chrono>
template <class ToDuration, class Rep, class Period>
constexpr ToDuration round(const duration<Rep, Period>& d);
(since C++17)

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 an instance of std::chrono::duration and std::chrono::treat_as_floating_point<typename ToDuration::rep>::value is false


d - duration to convert

Return value

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

Possible implementation

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>{} &&
                 !std::chrono::treat_as_floating_point<typename To::rep>{}>>
constexpr To round(const std::chrono::duration<Rep, Period>& d)
    To t0 = std::chrono::duration::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;


See also

converts a duration to another, with a different tick interval
(function template)
converts a duration to another, rounding down
(function template)
converts a duration to another, rounding up
(function template)
converts a time_point to another, rounding to nearest, ties to even
(function template)
nearest integer, rounding away from zero in halfway cases