code: function(d) {
      if (d.wy) return d.wy;
  
      // move to the Thursday in the target week and find Thurs of target year
      var wThur = later.dw.next(later.wy.start(d), 5),
          YThur = later.dw.next(later.Y.prev(wThur, later.Y.val(wThur)-1), 5);
  
      // caculate the difference between the two dates in weeks
      return (d.wy = 1 + Math.ceil((wThur.getTime() - YThur.getTime()) / later.WEEK));
    }
docs: The ISO week year value of the specified date.
  
  @param {Date} d: The date to calculate the value of