code: function leapdays(y1, y2) {
        y1--;
        y2--;
        return (Math.floor(y2/4) - Math.floor(y1/4)) - (Math.floor(y2/100) - Math.floor(y1/100)) + (Math.floor(y2/400) - Math.floor(y1/400));
      }
docs: Return number of leap years in range [y1, y2).
  Assumes y1 <= y2.
  
  @param {Number} y1
  @param {Number} y2
  @api public