code: function(d, val) {
      var month = later.date.prevRollover(d, val, later.dc, later.M),
          dcMax = later.dc.extent(month)[1];
  
      val = val > dcMax ? dcMax : val || dcMax;
  
      return later.dc.end(later.date.prev(
        later.Y.val(month),
        later.M.val(month),
        1 + (7 * (val - 1))
      ));
    }
docs: Returns the closest previous date with the day of week count specified.
  
  @param {Date} d: The starting date
  @param {int} val: The desired value, must be within extent