%j represents the day number of the year when formatting and parsing time_points. It is also handy to interpret this flag consistently when formatting and parsing durations. That is if there is not enough information in the stream to represent a time_point, and if the target of the format/parse is a duration, %j represents a number of days.

#include <chrono>
#include <iostream>
#include <sstream>
#include <string>

int main()
{
  using namespace std;
  using namespace std::chrono;
  // Parse %j as number of days into a duration
  istringstream in{"222"};
  hours d;
  in >> parse("%j", d);
  cout << d << '\n';
  cout << format("{:%j}", d) << '\n';
}

Output:

5328h
222