This class is designed for easily formatting dates, and parsing strings into dates. Example usage :

var now:Date = new Date();
 
var nowString:String =  SimpleDateFormatter.formatDate( new Date(), "yyyy-MMM-dd HH:mm:ssa" );
trace( nowString );
 
var nowAgain:Date =  SimpleDateFormatter.getDateFromFormat( nowString, "yyyy-MMM-dd HH:mm:ssa" );
trace( nowAgain );

The class :

/**
 * SimpleDateFormatter.as
 * 
 * An Actionscript 2 implementation of the Java SimpleDateFormat class.
 * This code was directly adapted from Matt Kruse's Javascript class
 * implementation, and is used/distributed with Matt's permission.
 * 
 * Please report all bugs to Daniel Wabyick (dwabyick@fluid.com).
 * 
 * @author Daniel Wabyick (Actionscript 2 port) 
 * 					   http://www.fluid.com
 * @author Matt Kruse ( Javascript implementation)
 * 					   http://www.JavascriptToolbox.com
 * 
 * The following notice is maintained from Matt Kruse's 
 * original Javascript code. 
 * 
 * NOTICE: You may use this code for any purpose, commercial or
 * private, without any further permission from the author. You may
 * remove this notice from your final code if you wish, however it is
 * appreciated by the author if at least my web site address is kept.
 *
 
 * HISTORY
 * ------------------------------------------------------------------
 * Oct 05, 2005 Wrapped into a static AS2 class - DWABYICK/FLUID
 * May 17, 2003: Fixed bug in parseDate() for dates <1970
 * March 11, 2003: Added parseDate() function
 * March 11, 2003: Added "NNN" formatting option. Doesn't match up
 *                 perfectly with SimpleDateFormat formats, but 
 *                 backwards-compatability was required.
 *
 * USAGE
 * ------------------------------------------------------------------
 * These functions use the same 'format' strings as the 
 * java.text.SimpleDateFormat class, with minor exceptions.
 * The format string consists of the following abbreviations:
 * 
 * Field        | Full Form          | Short Form
 * -------------+--------------------+-----------------------
 * Year         | yyyy (4 digits)    | yy (2 digits), y (2 or 4 digits)
 * Month        | MMM (name or abbr.)| MM (2 digits), M (1 or 2 digits)
 *              | NNN (abbr.)        |
 * Day of Month | dd (2 digits)      | d (1 or 2 digits)
 * Day of Week  | EE (name)          | E (abbr)
 * Hour (1-12)  | hh (2 digits)      | h (1 or 2 digits)
 * Hour (0-23)  | HH (2 digits)      | H (1 or 2 digits)
 * Hour (0-11)  | KK (2 digits)      | K (1 or 2 digits)
 * Hour (1-24)  | kk (2 digits)      | k (1 or 2 digits)
 * Minute       | mm (2 digits)      | m (1 or 2 digits)
 * Second       | ss (2 digits)      | s (1 or 2 digits)
 * AM/PM        | a                  |
 *
 * NOTE THE DIFFERENCE BETWEEN MM and mm! Month=MM, not mm!
 * Examples:
 *  "MMM d, y" matches: January 01, 2000
 *                      Dec 1, 1900
 *                      Nov 20, 00
 *  "M/d/yy"   matches: 01/20/00
 *                      9/2/00
 *  "MMM dd, yyyy hh:mm:ssa" matches: "January 01, 2000 12:30:45AM"
 */
class SimpleDateFormatter
{
	
	static var MONTH_NAMES=new Array('January','February','March','April','May','June','July','August','September','October','November','December','Jan','Feb','Mar','Apr','May','Jun','Jul','Aug','Sep','Oct','Nov','Dec');
	static var DAY_NAMES=new Array('Sunday','Monday','Tuesday','Wednesday','Thursday','Friday','Saturday','Sun','Mon','Tue','Wed','Thu','Fri','Sat');
	static function LZ(x) {return(x<0||x>9?"":"0")+x}
	
	
	
	/**
	 * Convert a date object into a string using the given format.
	 * @param date The date value to convert.
	 * @param format The format of the date object. (e.g. "yyyy-MM-dd")
	 * @return The string value of the date.
	 */
	public static function formatDate( date:Date, format:String ):String 
	{
		format=format+"";
		var result="";
		var i_format=0;
		var c="";
		var token="";
		var y=date.getYear()+"";
		var M=date.getMonth()+1;
		var d=date.getDate();
		var E=date.getDay();
		var H=date.getHours();
		var m=date.getMinutes();
		var s=date.getSeconds();
		// var yyyy,yy,MMM,MM,dd,hh,h,mm,ss,ampm,HH,H,KK,K,kk,k;
		// Convert real date parts into formatted versions
		var value=new Object();
		if (y.length < 4) {y=""+(y-0+1900);}
		value["y"]=""+y;
		value["yyyy"]=y;
		value["yy"]=y.substring(2,4);
		value["M"]=M;
		value["MM"]=LZ(M);
		value["MMM"]=MONTH_NAMES[M-1];
		value["NNN"]=MONTH_NAMES[M+11];
		value["d"]=d;
		value["dd"]=LZ(d);
		value["E"]=DAY_NAMES[E+7];
		value["EE"]=DAY_NAMES[E];
		value["H"]=H;
		value["HH"]=LZ(H);
		if (H==0){value["h"]=12;}
		else if (H>12){value["h"]=H-12;}
		else {value["h"]=H;}
		value["hh"]=LZ(value["h"]);
		if (H>11){value["K"]=H-12;} else {value["K"]=H;}
		value["k"]=H+1;
		value["KK"]=LZ(value["K"]);
		value["kk"]=LZ(value["k"]);
		if (H > 11) { value["a"]="PM"; }
		else { value["a"]="AM"; }
		value["m"]=m;
		value["mm"]=LZ(m);
		value["s"]=s;
		value["ss"]=LZ(s);
		while (i_format < format.length) 
		{
			c=format.charAt(i_format);
			token="";
			while ((format.charAt(i_format)==c) && (i_format < format.length)) 
			{
				token += format.charAt(i_format++);
			}
			if (value[token] != null)
			{ 
				result=result + value[token]; 
			}
			else { result=result + token; }
		}
		return result;
	}
	
	/**
	 * Determine if the a given string value is a date in the given format.
	 * @param A string value representing a date.
	 * @param format The format of this date. (e.g. "yyyy-MM-dd")
	 * @return true if date string matches format of format string and
	 * is a valid date. Else returns false.
	 */
	public static function isDate( val:String, format:String ) 
	{
		var date=getDateFromFormat( val,format );
		if ( date==0 ) 
		{
			 return false; 
		}
		return true;
	}
	
	/**
	 *   Compare two date strings to see which is greater.
	 *   @param date1 A string representing the first date value. 
	 *   @param dateformat1 The format of the first date. (e.g. "yyyy-MM-dd")
	 *   @param date2 A string representing the second date value.
	 *   @param dateformat2 The format of the second date.(e.g. "yyyy-MM-dd")
	 *   @return  1 if date1 >date2; 0 if date2 > date; -1 if either date is an invalid format. 
	 */
	public static function compareDates( date1:String, dateformat1:String, date2:String, dateformat2:String ):Number
	{
		var d1=getDateFromFormat(date1,dateformat1);
		var d2=getDateFromFormat(date2,dateformat2);
		if (d1==0 || d2==0) 
		{
			return -1;
		}
		else if (d1 > d2) 
		{
			return 1;
		}
		return 0;
	}
	
	/**
	 * Get a date using the given format. If it does not match, it returns 0.
	 * @param val The string value to convert to a date
	 * @param format The format of the date object.
	 * @return The date in the given format, or null if the value doesn't match the given format.
	 */
	public static function getDateFromFormat( val:String, format:String ) {
		val=val+"";
		format=format+"";
		var i_val=0;
		var i_format=0;
		var c="";
		var token="";
		var token2="";
		var x,y;
		var now=new Date();
		var year=now.getYear();
		var month=now.getMonth()+1;
		var date=1;
		var hh=now.getHours();
		var mm=now.getMinutes();
		var ss=now.getSeconds();
		var ampm="";
		
		while (i_format < format.length) 
		{
			// Get next token from format string
			c=format.charAt(i_format);
			token="";
			while ((format.charAt(i_format)==c) && (i_format < format.length)) 
			{
				token += format.charAt(i_format++);
			}
			// Extract contents of value based on format token
			if (token=="yyyy" || token=="yy" || token=="y") 
			{
				if (token=="yyyy") { x=4;y=4; }
				if (token=="yy")   { x=2;y=2; }
				if (token=="y")    { x=2;y=4; }
				year=_getInt(val,i_val,x,y);
				if (year==null) { return null; }
				i_val += year.length;
				if (year.length==2) 
				{
					if (year > 70) { year=1900+(year-0); }
					else { year=2000+(year-0); }
				}
			}
			else if (token=="MMM"||token=="NNN")
			{
				month=0;
				for (var i=0; i<MONTH_NAMES.length; i++) 
				{
					var month_name=MONTH_NAMES[i];
					if (val.substring(i_val,i_val+month_name.length).toLowerCase()==month_name.toLowerCase())
					{
						if (token=="MMM"||(token=="NNN"&&i>11)) 
						{
							month=i+1;
							if (month>12) { month -= 12; }
							i_val += month_name.length;
							break;
						}
					}
				}
				if ((month < 1)||(month>12)){return null;}
			}
			else if (token=="EE"||token=="E")
			{
				for (var i=0; i<DAY_NAMES.length; i++) 
				{
					var day_name=DAY_NAMES[i];
					if (val.substring(i_val,i_val+day_name.length).toLowerCase()==day_name.toLowerCase()) 
					{
						i_val += day_name.length;
						break;
					}
				}
			}
			else if (token=="MM"||token=="M") 
			{
				month=_getInt(val,i_val,token.length,2);
				if(month==null||(month<1)||(month>12)){return null;}
				i_val+=month.length;
				}
			else if (token=="dd"||token=="d") 
			{
				date=_getInt(val,i_val,token.length,2);
				if(date==null||(date<1)||(date>31)){return null;}
				i_val+=date.length;
				}
			else if (token=="hh"||token=="h") 
			{
				hh=_getInt(val,i_val,token.length,2);
				if(hh==null||(hh<1)||(hh>12)){return null;}
				i_val+=hh.length;
				}
			else if (token=="HH"||token=="H") 
			{
				hh=_getInt(val,i_val,token.length,2);
				if(hh==null||(hh<0)||(hh>23)){return null;}
				i_val+=hh.length;
			}
			else if (token=="KK"||token=="K") 
			{
				hh=_getInt(val,i_val,token.length,2);
				if(hh==null||(hh<0)||(hh>11)){return null;}
				i_val+=hh.length;
			}
			else if (token=="kk"||token=="k") 
			{
				hh=_getInt(val,i_val,token.length,2);
				if(hh==null||(hh<1)||(hh>24)){return null;}
				i_val+=hh.length;hh--;
			}
			else if (token=="mm"||token=="m") 
			{
				mm=_getInt(val,i_val,token.length,2);
				if(mm==null||(mm<0)||(mm>59)){return null;}
				i_val+=mm.length;
			}
			else if (token=="ss"||token=="s") 
			{
				ss=_getInt(val,i_val,token.length,2);
				if(ss==null||(ss<0)||(ss>59)){return null;}
				i_val+=ss.length;
			}
			else if (token=="a") 
			{
				if (val.substring(i_val,i_val+2).toLowerCase()=="am") {ampm="AM";}
				else if (val.substring(i_val,i_val+2).toLowerCase()=="pm") {ampm="PM";}
				else {return null;}
				i_val+=2;
			}
			else 
			{
				if (val.substring(i_val,i_val+token.length)!=token) {return null;}
				else {i_val+=token.length;}
			}
		}
		// If there are any trailing characters left in the value, it doesn't match
		if (i_val != val.length) { return null; }
		// Is date valid for month?
		if (month==2) 
		{
			// Check for leap year
			if ( ( (year%4==0)&&(year%100 != 0) ) || (year%400==0) ) 
			{ // leap year
				if (date > 29){ return null; }
			}
			else { if (date > 28) { return null; } }
		}
		if ((month==4)||(month==6)||(month==9)||(month==11)) 
		{
			if (date > 30) { return null; }
		}
		// Correct hours value
		if (hh<12 && ampm=="PM") { hh=hh-0+12; }
		else if (hh>11 && ampm=="AM") { hh-=12; }
		var newdate = new Date(year,month-1,date,hh,mm,ss);
		return newdate;
	}
	
	
		
	/**
	 * @return True if the value is an integer; false otherwise.
	 */
	private static function _isInteger(val) : Boolean 
	{
		var digits="1234567890";
		for (var i=0; i < val.length; i++) 
		{
			if (digits.indexOf(val.charAt(i))==-1) 
			{ 
				return false; 
			}
		}
		return true;
	}
	
 
	private static function _getInt(str,i,minlength,maxlength) : Number 
	{
		for (var x=maxlength; x>=minlength; x--) 
		{
			var token=str.substring(i,i+x);
			if (token.length < minlength) 
			{
				return null; 
			}
			if (_isInteger(token)) 
			{ 
				return token; 
			}
		}
		return null;
	}
}