docs: function () {
          // days between midweek of this week and jan 4
          // (f.z() - f.N() + 1 + 3.5) - 3
          var midWeekDaysFromJan4 = f.z() - f.N() + 1.5;
          // 1 + number of weeks + rounded week
          return humanize.pad(1 + Math.floor(Math.abs(midWeekDaysFromJan4) / 7) + (midWeekDaysFromJan4 % 7 > 3.5 ? 1 : 0), 2, '0');
        }