std::chrono::duration has two template parameters, the second being exactly the unit of measure. You can invoke std::chrono::duration_cast to cast from one duration type to another. Also, there is a predefined duration type for milliseconds: std::chrono::milliseconds. Composing this together:
auto milliseconds = std::chrono::duration_cast<std::chrono::milliseconds>(foo - now);
To get the actual number of milliseconds, use duration::count:
auto ms = milliseconds.count();
Its return type is duration::rep, which for standard duration types like std::chrono::milliseconds is a signed integer of unspecified size.