not working javascript age validation

<script type="text/javascript">
function checkDate(sender,args)
{
//create a new date var and set it to the
//value of the senders selected date
var selectedDate = new Date();
selectedDate = sender._selectedDate;
//create a date var and set it's value to today
var todayDate = new Date();
var mssge = "";

if(DateTime.UtcNow.CompareTo(selectedDate) >17)
 {
 alert("Warning! - Date of Birth  is  valid  ");
 }

}
</script>
            <td >
                Date Of&nbsp; Birth</td>
            <td style="width: 292px">

            <asp:TextBox id="TextBox3" runat="server"  ValidationGroup="a"></asp:TextBox>
 <asp:calendarextender id="CalendarExtender2" targetcontrolid="TextBox3"  runat="server"  OnClientDateSelectionChanged="checkDate" >
</asp:calendarextender>
  <%--<asp:CustomValidator ID="CustomValidator1" runat="server" ControlToValidate="TextBox3" ErrorMessage="CustomValidator"
          ClientValidationFunction="CustomValidator1_ServerValidate"  Display="Dynamic" ValidationGroup="a"></asp:CustomValidator>--%>
                <asp:ValidatorCalloutExtender ID="ValidatorCalloutExtender3" runat="server" TargetControlID="RequiredFieldValidator3">
                </asp:ValidatorCalloutExtender>

                <asp:RequiredFieldValidator ID="RequiredFieldValidator3" runat="server"
                    ErrorMessage="RequiredFieldValidator"  Display="None"
                    ControlToValidate="TextBox3" SetFocusOnError="True" ValidationGroup="a"></asp:RequiredFieldValidator>


            </td>

        </tr>
        <tr>

                <asp:Button ID="Button1" runat="server" Text="Insert"  ValidationGroup="a" onclick="Button1_Click"
                    style="width: 48px"  />