C++ chrono system time in milliseconds, time operations

I've got a small problem caused by insufficient documentation of C++11.

I'd like to obtain a time since epoch in milliseconds, or nanoseconds or seconds and then I will have to "cast" this value to another resolution. I can do it using gettimeofday() but it will be to easy, so I tried to achieve it using std::chrono.

I tried:

std::chrono::time_point<std::chrono::system_clock> now = 
    std::chrono::system_clock::now();

But I have no idea what is a resolution of obtained in this way time_point, and I don't know how to get this time as a simple unsigned long long, and I haven't any conception how to cast it to another resolution.

You can do now.time_since_epoch() to get a duration representing the time since the epoch, with the clock's resolution. To convert to milliseconds use duration_cast:

auto duration = now.time_since_epoch();
auto millis = std::chrono::duration_cast<std::chrono::milliseconds>(duration).count();