Automatically add one year to Selected Date of AJAX Calendar using JavaScript in ASP.Net

<cc1:ToolkitScriptManager ID="ToolkitScriptManager1" runat="server">
</cc1:ToolkitScriptManager>
<asp:TextBox ID="TextBox1" runat="server" />
<asp:ImageButton runat="server" ID="ImageButton1" ImageUrl="~/Calendar.png" />
<cc1:CalendarExtender ID="CalendarExtender1" runat="server" TargetControlID="TextBox1"
    PopupButtonID="ImageButton1" OnClientDateSelectionChanged="OnClientDateSelectionChanged" />
<script type="text/javascript">
    function OnClientDateSelectionChanged(sender, args) {
        $find("<%=CalendarExtender2.ClientID %>").set_startDate(sender._selectedDate);
    }
</script>
<asp:TextBox ID="TextBox2" runat="server" />
<asp:ImageButton runat="server" ID="ImageButton2" ImageUrl="~/Calendar.png" />
<cc1:CalendarExtender ID="CalendarExtender2" runat="server" TargetControlID="TextBox2"
    PopupButtonID="ImageButton2" />

this is the code i wnt to add 1 year here Sender._SelectedDate

<script type="text/javascript"> function OnClientDateSelectionChanged(sender, args) { $find("<%=CalendarExtender2.ClientID %>").set_startDate(sender._selectedDate); } </script>