OnValueChangeClientScript Issue in SharePoint DatetimeControl

Hi,

I am using following SharePoint datetime control and i want to compare user selected date with the current date. for this i am calling a javascript function on "OnValueChangeClientScript:".

but the problem is alert('Please Select current Date or Past Date') is called two times everytime. i couldn't figure it out why it is calling two times. any suggestions ??

<SharePoint:DateTimeControl ID="dtTimeCtrl" runat="server" OnValueChangeClientScript="updateDates()"></SharePoint:DateTimeControl>

javascript code:

<script type="text/javascript">
function updateDates(){
var userDate = document.getElementById('<%=DateTimeControl1.Controls[0].ClientID%>').value;

       var today = new Date();
var dd = today.getDate();
var mm = today.getMonth()+1;//January is 0!`

var yyyy = today.getFullYear();
if(dd<10){dd='0'+dd}
if(mm<10){mm='0'+mm}
var today = mm+'/'+dd+'/'+yyyy;

            if(new Date(userDate).getTime() > new Date(today).getTime()) 
            { 
            document.getElementById('<%=DateTimeControl1.Controls[0].ClientID%>').value = "";
            alert('Please Select current Date or Past Date');
            return;
            }        
        }

</script>

Thanks

Bob