Is it well-formed to overload an operator for a standard-defined type in global namespace that doesn't depend on a user-define type? [duplicate]

Question

Is the following code well-formed program per the standard?

std::ostream& operator<<(std::ostream& out, std::chrono::system_clock::time_point tp)
{
    return out << tp.time_since_epoch().count();
}

This isn't a template defined in the standard library, and isn't defined inside the namespace std, so it seems to be well-formed, but I might me missing something.

Answer 1

I don't see anything in the section named Constraints on programs that forbids defining a function like yours.

I say, it is perfectly valid to do so.