flex验证日期

<mx:FormItem label="日历控件">
	<mx:DateField id="txt_df1" formatString="YYYY-MM-DD" width="200"/>
</mx:FormItem>
<mx:FormItem label="日历控件">
	<mx:DateField id="txt_df2" formatString="YYYY-MM-DD" width="200"/>
</mx:FormItem>

var d1:Number =  Date.parse(txt_df1.selectedDate);
var d2:Number =  Date.parse(txt_df2.selectedDate);
	if(d1 >= d2) {
		Alert.show("结束日期必须大于开始日期");
	}