The datechooser up to Flash 8 has a bug. Click on each on the left datechooser on the same date twice. When you clicked the second time there will be undefined. On the right side the datechooser has been fixed adding some lines to the script.
To fix the issue we create variables, which remember the date, when it was not undefined. 
// Variables to remember
var rememberDate:Number;
var rememberMonth:Number;
var rememberYear:Number;
// regular Variables
var theDate:Number;
var theMonth:Number;
var theYear:Number;
// Listener
var myDateListener:Object = new Object ();
myDateListener.change = function (eventObj)
{
// We fill the remember variables with a value.
	if (theDate != undefined)
	{
		rememberDate = theDate;
		rememberMonth = theMonth;
		rememberYear = theYear;
	}
	// Now we get the data from the component.
	var eventSource:Object = eventObj.target;
	var theSelectedDate:Date = eventSource.selectedDate;
	theDate = theSelectedDate.getDate ();
	theMonth = theSelectedDate.getMonth () + 1;
	theYear = theSelectedDate.getFullYear ();
	// If any of the data are undefined, we use the remember variables as data source.
	// The error only occurs, when the same date was pressed twice.
	if (theDate == undefined)
	{
		theDate = rememberDate;
		theMonth = rememberMonth;
		theYear = rememberYear;
	}
	myText.htmlText = "
" + theMonth + "-" + theDate + "-" + theYear + "
";
}
myDateChooser.addEventListener ("change",myDateListener2);