code: function(d) {
      var year = later.Y.val(d);
  
      // shortcut on finding leap years since this function gets called a lot
      // works between 1901 and 2099
      return d.dyExtent || (d.dyExtent = [1, year % 4 ? 365 : 366]);
    }
docs: The minimum and maximum valid day of year values of the month specified.
  Zero indicates the last day of the year.
  
  @param {Date} d: The date indicating the month to find the extent of