code: function getWeekNumber(date) {
      date = date.clone();
      var dow = callDateGet(date, 'Day') || 7;
      date.addDays(4 - dow).reset();
      return 1 + floor(date.daysSince(date.clone().beginningOfYear()) / 7);
    }
docs: weeksSince won't work here as the result needs to be floored, not rounded.