set the date back to the current date in ajax calender extender using javascript

In need to disable previous date selection from ajax calender extender.

<asp:TextBox ID="txtFromDate" runat="server" CssClass="text_box left w80"></asp:TextBox><asp:ImageButton ID="fromDatePicker" CssClass="datepicker" runat="server" CausesValidation="false"
            ImageUrl="~/images/datepicker.png"></asp:ImageButton><ajaxToolkit:CalendarExtender ID="fromDateCalendarExtender" runat="server" EnabledOnClient="true"
            PopupButtonID="fromDatePicker" TargetControlID="txtFromDate" OnClientDateSelectionChanged="checkForPreviousDate"></ajaxToolkit:CalendarExtender>

I am using the below given js function

<script type="text/javascript"> 
        // Do not move this to top script tag..it wil throw js error if moved to top 
            function checkForPreviousDate(sender, args) { 
                if (sender._selectedDate < new Date()) { 
                    alert("You cannot select a day earlier than today!"); 
                    sender._selectedDate = new Date(); 
                    // set the date back to the current date 
                    sender._textbox.set_Value(sender._selectedDate.format(sender._format)); 
                } 
            }  </script>

But the code to set the date back to current date is not working and it gives a js error- sender._textbox is undefined.

 sender._textbox.set_Value(sender._selectedDate.format(sender._format));

Any idea to set the current date and remove the js error?