Filter ASP.Net GridView with Year Month and Day using C#

hello,

i would like filter by year/ month

there is a simple guide like this Filter DataTable and Bind to GridView in ASP.Net using C# and VB.Net 

i want change country to datetimepicker, so if i want filter by year / month ( 2020 / 5 ) 

but when i use my code, i get different

when i  select 1-8-2021 i get from 5-8-2021

when i select 12-8-2021 i get also 5-8-2021

c#

        protected void OnSearch(object sender, EventArgs e)
        {
            string dob = Request.Form[txtBoxDatestart.UniqueID];
            DataTable dt = this.GetData("SELECT * FROM Table_vacation");
            if (!string.IsNullOrEmpty(dob))
            {
                DateTime selectedDate = Convert.ToDateTime(dob);
                string query = string.Format("SELECT * FROM Table_vacation WHERE DATEPART(YEAR,dateend) = {0} AND DATEPART(MONTH,dateend) = {1}",
                                            selectedDate.Year, selectedDate.Month);
                dt = this.GetData(query);
            }
            gvFiles.DataSource = dt;
            gvFiles.DataBind();
            txtBoxDatestart.Text = dob;
        }

        private DataTable GetData(string query)
        {
            DataTable dt = new DataTable();
            string constr = ConfigurationManager.ConnectionStrings["kankonConnectionString"].ConnectionString;
            using (SqlConnection conn = new SqlConnection(constr))
            {
                using (SqlCommand cmd = new SqlCommand(query))
                {
                    cmd.Connection = conn;
                    using (SqlDataAdapter sda = new SqlDataAdapter(cmd))
                    {
                        sda.Fill(dt);
                    }
                }
            }

            return dt;
        }

<asp:TextBox ID="txtBoxDatestart" runat="server" AutoPostBack="True" 
    BackColor="#99CCFF" BorderColor="#99CCFF" CssClass="auto-style182" 
    Font-Bold="True" Font-Names="Arial" Font-Size="Medium" 
    Style="text-align: center"></asp:TextBox>

     <script type="text/javascript">
         $(function () {
             $("#<%= txtBoxDatestart.ClientID %>").datepicker({
                 dateFormat: 'yy/mm/dd',
                 changeMonth: true,
                 changeYear: true,
                 yearRange: '2015:2035',
                showButtonPanel: true,
                beforeShowDay: function (date) {
                    // Sets Weekend Friday and Saturday.
                    var weekend = date.getDay() == 5 || date.getDay() == 6;
                    return [!weekend, 'myweekend'];
                },
                // Sets Weekend position.
                firstDay: 6,
                closeText: "إغلاق",
                prevText: "&#x3C;السابق",
                nextText: "التالي&#x3E;",
                currentText: "اليوم",
                monthNames: ["يناير", "فبراير", "مارس", "أبريل", "مايو", "يونيو", "يوليو", "أغسطس", "سبتمبر", "أكتوبر", "نوفمبر", "ديسمبر"],
                monthNamesShort: ["1", "2", "3", "4", "5", "6", "7", "8", "9", "10", "11", "12"],
                dayNames: ["الأحد", "الاثنين", "الثلاثاء", "الأربعاء", "الخميس", "الجمعة", "السبت"],
                dayNamesShort: ["ح", "ن", "ث", "ر", "خ", "ج", "س"],
                dayNamesMin: ["أحد", "اثنين", "ثلاثاء", "أربعاء", "خميس", "جمعة", "سبت"],
                weekHeader: "أسبوع",
                isRTL: false,
                showMonthAfterYear: false,
                yearSuffix: ""
            });
        });
     </script>

 <asp:Button Text="Search" runat="server" OnClick="OnSearch" CssClass="button" />