A `std::chrono::duration` represents a length of time. Its template parameters specify the count’s representation type and the size of each unit in seconds: `std::chrono::milliseconds` is the predefined duration for millisecond counts. section 4.3.2
Predefined duration types include nanoseconds, microseconds, milliseconds, seconds, minutes, and hours. C++14 chrono literals offer concise values such as `30ms`; `duration_cast` converts a duration to another unit. section 4.3.2
std::chrono::milliseconds ms(54802);
std::chrono::seconds s=
std::chrono::duration_cast<std::chrono::seconds>(ms);
A millisecond count is explicitly converted to seconds with `duration_cast`. section 4.3.2
Durations also specify timeouts for operations such as waiting on futures or condition variables and trying to acquire timed mutexes. section 4.3.4
std::future<int> f=std::async(some_task);
if(f.wait_for(std::chrono::milliseconds(35))==std::future_status::ready)
do_something_with(f.get());
This example passes a 35-millisecond duration to `future::wait_for` and checks whether the result is ready. section 4.3.4
