Proposed resolution:

This wording is relative to N4830.

1. Modify "Table 98 — Meaning of conversion specifiers" [tab:time.format.spec] as indicated:

   Table 98 — Meaning of conversion specifiers [tab:time.format.spec]

   Specifier	Replacement
   […]
   %j	If the type being formatted is a specialization of duration, the decimal number of days without padding. Otherwise, theThe day of the year as a decimal number. Jan 1 is 001. If the result is less than three digits, it is left-padded with 0 to three digits.
   […]

2. Modify "Table 99 — Meaning of parse flags" [tab:time.parse.spec] as indicated:

   Table 99 — Meaning of parse flags [tab:time.parse.spec]

   Flag	Parsed value
   […]
   %j	If the type being parsed is a specialization of duration, a decimal number of days. Otherwise, theThe day of the year as a decimal number. Jan 1 is 1. In either case, theThe modified command %Nj specifies the maximum number of characters to read. If N is not specified, the default is 3. Leading zeroes are permitted but not required.
   […]

[ 2020-02 Status to Immediate on Thursday night in Prague. ]

[ 2020-02-13 After Thursday afternoon discussion in Prague, Marshall provides updated wording. ]

[ 2019-10 Priority set to 2 after reflector discussion ]

Previous resolution [SUPERSEDED]:

This wording is relative to N4830.

1. Modify "Table 98 — Meaning of conversion specifiers" [tab:time.format.spec] as indicated:

   Table 98 — Meaning of conversion specifiers [tab:time.format.spec]

   Specifier	Replacement
   […]
   %j	The day of the year as a decimal number. Jan 1 is 001. If the result is less than three digits, it is left-padded with 0 to three digits. If the type being formatted is a specialization of duration, it is formatted as a decimal number of days.
   […]

2. Modify "Table 99 — Meaning of parse flags" [tab:time.parse.spec] as indicated:

   Table 99 — Meaning of parse flags [tab:time.parse.spec]

   Flag	Parsed value
   […]
   %j	The day of the year as a decimal number. Jan 1 is 1. The modified command %Nj specifies the maximum number of characters to read. If N is not specified, the default is 3. Leading zeroes are permitted but not required. If the type being parsed is a specialization of duration, it is parsed as a decimal number of days.
   […]

%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