Ajax Calander Controler date difference

Sir,

I have two AJAX Calander Controls on my form and I want to calculate difference of days between these to controls

<asp:TextBox ID="txtstdate" runat="server" Height="22px" Width="109px"></asp:TextBox><asp:CalendarExtender ID="txtstdate_CalendarExtender" runat="server" 
                            TargetControlID="txtstdate"></asp:CalendarExtender><asp:TextBox ID="txtenddate" runat="server" Height="22px" Width="110px"  onblur="btndadays_onclick()"></asp:TextBox><asp:CalendarExtender ID="txtenddate_CalendarExtender" runat="server" 
                            Enabled="True" TargetControlID="txtenddate"></asp:CalendarExtender>

I have written a Javascript on Blur of second date.

<script language="javascript" type="text/javascript">
        function btndadays_onclick() {
            var st1 = document.getElementById("<%= txtstdate.ClientId %>").value;
            var ed1 = document.getElementById("<%= txtenddate.ClientId %>").value;

            var timeDiff = Math.abs(ed1.getTime() - st1.getTime());
            var diffDays = Math.ceil(timeDiff / (1000 * 3600 * 24));
            document.getElementById("<%= txtdays.ClientId %>").value= diffdays ;
        }</script>

But I am having error as

Math.abs(ed1.getTime())