+template <typename To, typename Diff>
+constexpr To round_impl(To const& t0, To const& t1, Diff diff0, Diff diff1) {
+ return diff0 < diff1 ? t0 : diff1 < diff0 ? t1 : t0.count() & 1 ? t1 : t0;
+}
+
+template <typename To, typename Duration>
+constexpr To round_impl(Duration const& d, To const& t0, To const& t1) {
+ return round_impl(t0, t1, d - t0, t1 - d);
+}
+
+template <typename To, typename Duration>
+constexpr To round_impl(Duration const& d, To const& t0) {
+ return round_impl(d, t0, t0 + To{1});
+}
+} // namespace detail
+
+// mimic: std::chrono::ceil, C++17
+// from: http://en.cppreference.com/w/cpp/chrono/duration/ceil, CC-BY-SA
+template <
+ typename To,
+ typename Rep,
+ typename Period,
+ typename = typename std::enable_if<detail::is_duration<To>::value>::type>
+constexpr To ceil(std::chrono::duration<Rep, Period> const& d) {
+ return detail::ceil_impl(d, std::chrono::duration_cast<To>(d));
+}
+
+// mimic: std::chrono::ceil, C++17
+// from: http://en.cppreference.com/w/cpp/chrono/time_point/ceil, CC-BY-SA
+template <
+ typename To,
+ typename Clock,
+ typename Duration,
+ typename = typename std::enable_if<detail::is_duration<To>::value>::type>
+constexpr std::chrono::time_point<Clock, To> ceil(
+ std::chrono::time_point<Clock, Duration> const& tp) {
+ return std::chrono::time_point<Clock, To>{ceil<To>(tp.time_since_epoch())};
+}
+
+// mimic: std::chrono::floor, C++17
+// from: http://en.cppreference.com/w/cpp/chrono/duration/floor, CC-BY-SA
+template <
+ typename To,
+ typename Rep,
+ typename Period,
+ typename = typename std::enable_if<detail::is_duration<To>::value>::type>
+constexpr To floor(std::chrono::duration<Rep, Period> const& d) {
+ return detail::floor_impl(d, std::chrono::duration_cast<To>(d));
+}
+
+// mimic: std::chrono::floor, C++17
+// from: http://en.cppreference.com/w/cpp/chrono/time_point/floor, CC-BY-SA
+template <
+ typename To,
+ typename Clock,
+ typename Duration,
+ typename = typename std::enable_if<detail::is_duration<To>::value>::type>
+constexpr std::chrono::time_point<Clock, To> floor(
+ std::chrono::time_point<Clock, Duration> const& tp) {
+ return std::chrono::time_point<Clock, To>{floor<To>(tp.time_since_epoch())};
+}
+
+// mimic: std::chrono::round, C++17
+// from: http://en.cppreference.com/w/cpp/chrono/duration/round, CC-BY-SA
+template <
+ typename To,
+ typename Rep,
+ typename Period,
+ typename = typename std::enable_if<
+ detail::is_duration<To>::value &&
+ !std::chrono::treat_as_floating_point<typename To::rep>::value>::type>
+constexpr To round(std::chrono::duration<Rep, Period> const& d) {
+ return detail::round_impl(d, floor<To>(d));
+}
+
+// mimic: std::chrono::round, C++17
+// from: http://en.cppreference.com/w/cpp/chrono/time_point/round, CC-BY-SA
+template <
+ typename To,
+ typename Clock,
+ typename Duration,
+ typename = typename std::enable_if<
+ detail::is_duration<To>::value &&
+ !std::chrono::treat_as_floating_point<typename To::rep>::value>::type>
+constexpr std::chrono::time_point<Clock, To> round(
+ std::chrono::time_point<Clock, Duration> const& tp) {
+ return std::chrono::time_point<Clock, To>{round<To>(tp.time_since_epoch())};
+}
+} // namespace chrono
+} // namespace folly
+
+#endif