使い方は以下のようにします。

var rssEx:RssDateExchange = new RssDateExchange("Thu, 08 Oct 2009 14:34:01 +0900");
trace(rssEx.start());

でトレースした結果は→「　2009/10/8 (木)　」です。

・ソース

package
{
	
	public class RssDateExchange 
	{
		private var dateList:Array = new Array();;
		private var monthList:Array = new Array();
		private var dayList:Array = new Array();
		private var jDayList:Array = new Array();
		private var gmt:Number;
		private var n:Number;
		public static var dateStyle:String;
		
		public function RssDateExchange(date:String)
		{
			dateList = date.split(" ");
			trace(dateList)
		}
		
		public function start():String {
			
			monthList = ["Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"];
			dayList = ["Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat"];
			jDayList = ["日", "月", "火", "水", "木", "金", "土"];
			
			var year:Number = new Number(dateList[3]);
			for (var n:Number = 0; n < monthList.length; n++) {
				if (dateList[2] == monthList[n]) {
					var month:Number = n;
					break;
				}
			}
			
			var date:Number = new Number(dateList[1]);
			for (var n1:Number = 0; n1 < dayList.length; n1++) {
				var day1:String = dateList[0].substr(0, 3);
				if (day1 == dayList[n]) {
					var day:Number = n;
					break;
				}
			}
			
			var timeList:Array = dateList[4].split(":");
			var hour:Number = strToNumber(timeList[0]);
			var GMT:Number = gmtToNumber(dateList[5]);
			var minute:Number = strToNumber(timeList[1]);
			var second:Number = strToNumber(timeList[2]);
			var d:Date = new Date(year, month, date, hour+GMT, minute, second);
			dateStyle = d.getFullYear()+"/"+(d.getMonth()+1)+"/"+d.getDate()+" ("+jDayList[d.getDay()]+")";
			return dateStyle;
		}
		
		private function strToNumber(str:String):Number {
			
			if (str.substr(0, 1) == "0") {
				n = Number(str.substr(1, 1));
			} else {
				n = Number(str.substr(0, 2));
			}
			return n;
		}
		
		private function gmtToNumber(str:String):Number {
			str = str.substr(0, 3);
			var n:Number = strToNumber(str.substr(1, 2));
			if (str.substr(0, 1) == "+") {
				gmt = n;
			} else {
				gmt = -n;
			}
			return gmt;
		}
		
	}
}