You can get the date straight from the DateFormatter. For example, getting current Date & time to String:
//..
var todaysDate:String = getCurrentDate();
//..
private function getCurrentDate():String{
var CurrentDateTime:Date = new Date();
var CurrentDF:DateFormatter = new DateFormatter();
CurrentDF.formatString = "MM/DD/YY LL:NN:SS A"
var DateTimeString:String = CurrentDF.format(CurrentDateTime);
return DateTimeString;
}
So in your case you could use this as formatString:
CurrentDF.formatString = "MM_DD_YY_LL_NN_SS_A"
so your string would look like 12_06_2013_12_56_AM.