Javascript String to_date()

String.prototype.to_date = function(){
// don't quite understand it, but I need to convert this string to a string?
// otherwise it is invalid
 d = new Date(this.toString());
 if( d == "Invalid Date" ) {
  if(this.length == 8){  /* 19991231 */
   d = new Date(this.substr(0,4), this.substr(4,2).to_i()-1, this.substr(6,2));
  }/*from w ww  .j a  v a  2s . com*/
 }
 return d;
}