/**
* Converts millisecond figure into timecode
*	@param	millisecond		total number of milliseconds - accurate up to one hour
*/
public static function toTimeCode(milliseconds:int) : String {
	var isNegative:Boolean = false;
	if (milliseconds < 0) {
		isNegative = true;
		milliseconds = Math.abs(milliseconds);			
	}
	
	var seconds:int = Math.round((milliseconds/1000) % 60);
	var strSeconds:String = (seconds < 10) ? ("0" + String(seconds)) : String(seconds);
	if(seconds == 60) strMinutes = "00";
	var minutes:int = Math.round(Math.floor((milliseconds/1000)/60));
	var strMinutes:String = (minutes < 10) ? ("0" + String(minutes)) : String(minutes);
	
	if(minutes > 60) {
		strSeconds = "60";
		strMinutes = "00";
	}
	
	var timeCodeAbsolute:String = strMinutes + ":" + strSeconds;
	var timeCode:String = (isNegative) ? "-" + timeCodeAbsolute : timeCodeAbsolute;
	return timeCode;
} // end toTimeCode

Handy function for converting milliseconds to video timecode

Flash: convert milliseconds to timecode

flash

ActionScript 3