How Do I Set Start And End Month In Datepicker Which Shows Only Month And Year

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VB

ASP.NET

JScript.NET

I have two textbox from and to which shows month and year picker. I want to set mindate(month) for to textbox from the value from textbox which user has selected.
Here is my jscript but its not working.

$(function () {
    $("#<%=frm_txtdatefrm.ClientID %>").datepicker({
        changeMonth: true,
        changeYear: true,
        dateFormat: 'M yy',
        showButtonPanel: true,
        onClose: function () {
            var iMonth = $("#ui-datepicker-div .ui-datepicker-month :selected").val();

            var iYear = $("#ui-datepicker-div .ui-datepicker-year :selected").val();

            $(this).datepicker('setDate', new Date(iYear, iMonth, 1));
            $(this).datepicker('refresh');
        },

        beforeShow: function () {
            if ((selDate = $(this).val()).length > 0) {
                iYear = selDate.substring(selDate.length - 4, selDate.length);

                iMonth = jQuery.inArray(selDate.substring(0, selDate.length - 5), $(this).datepicker('option', 'monthNames'));

                $(this).datepicker('option', 'defaultDate', new Date(iYear, iMonth, 1));
                $(this).datepicker('setDate', new Date(iYear, iMonth, 1));
            }
            if ((selDate = $(this).val()).length > 0) {
                year = selectedDate.substring(selectedDate.length - 4, selectedDate.length);
                month = jQuery.inArray(selectedDate.substring(0, selectedDate.length - 5), $(this).datepicker('option', 'monthNamesShort'));
                $("#<%=frm_txtdateto.ClientID %>").datepicker("option", "mindate", new Date(year, month, 1));
            }
        }
    });

    $("#<%=frm_txtdatefrm.ClientID %>").focus(function () {
        $(".ui-datepicker-calendar").hide();
        $("#ui-datepicker-div").position({
            my: "center top",
            at: "center bottom",
            of: $(this)
        });
    });

    $("#<%=frm_txtdatefrm.ClientID %>").blur(function () {
        $(".ui-datepicker-calendar").hide();
    });