Why value of textbox change on selecting value from dropdown list

Good Ev everyone i am using jquery for my 2 textboxes 1.txtStartDate 2.txtEndDate , I have used first date of month as default date for firsttextbox and present date for second text box and on third dropdownList i have used Autopostback as :

<asp:DropDownList ID="ddlWorkType" CssClass="selectbox1 ddlWidth" ValidationGroup="tab1" runat="server" Enabled="true" AutoPostBack="true" AppendDataBoundItems="true" OnSelectedIndexChanged="ddlWorkType_SelectedIndexChanged1">

whenever i select date in first textbox and then select value from dropdownlist it reset the vaue of first textbox again to default

example : textbox1=12-09-2017 textbox2 = 11-10-2017 as i select value in third text box it reset value of textbox1 = 01-10-2017

What I have tried:

$(function () {

           $("#txtStartDate").datepicker({

               dateFormat: 'yy-mm-dd',
               showOn: 'button',
               buttonImageOnly: true,
               buttonImage: '/Image/calender.jpg',
               buttonText: ""
           }).bind("change", function () {
               var minValue = $(this).val();
               minValue = $.datepicker.parseDate("yy-mm-dd", minValue);
           })
           var d = new Date();
           var currMonth = d.getMonth();
           var currYear = d.getFullYear();
           var startDate = new Date(currYear, currMonth, 1);
           $('#txtStartDate').datepicker('setDate', startDate);
           $("#txtEndDate").datepicker({
               dateFormat: 'yy-mm-dd',
               showOn: 'button',
               buttonImageOnly: true,
               buttonImage: '/Image/calender.jpg',
               buttonText: ""
           }).bind("change", function () {
               var minValue = $(this).val();
               minValue = $.datepicker.parseDate("yy-mm-dd", minValue);

           })
           $('#txtEndDate').datepicker('setDate', new Date());
       });

Tags: C#, jQuery