How to add month in javascript using datepicker?

See more:

ASP.NET

Javascript

i have a problem with my project ,

I have two datepicker in my project ,


value of the second datepicker in setted auto based on first datepicker ,

example of ,

txt.datepicker value ='07/09/2020'

so txt2.datepicker value is txt.datepicker + 2 month
or '07/11/2020'

how can i solve this?

What I have tried:

this is my code

<asp:Content ID="BodyContent" ContentPlaceHolderID="MainContent" runat="server">
 <div class="box-body">
     <dl class="dl-horizontal">
        <dt style="text-align:left">
               <label for="StartDate">Start Date</label>
                    </dt>
                      <dd style="text-align:left" >
                         <asp:TextBox ID="txtDate" onkeypress="copyText()"  Width="200px" runat="server" ReadOnly="true" Height="34px"></asp:TextBox>
                        </dd>
                             <dt style="text-align:left">
                                 <label for="StartDate">Expired Date</label>
                                 </dt>
                                 <dd style="text-align:left">
                                       <asp:TextBox ID="txtdate2"  ReadOnly="true"  Width="200px" Height="34px" runat="server"></asp:TextBox>
                                    </dd>
         </dl>
         </div>
<script type="text/javascript">
         $(function () {
             $("[id*=txtDate]").datepicker({
                 showOn: 'button',
                 buttonImageOnly: true,
                 buttonImage: '/Images/calendar.png',
             });
             $(".ui-datepicker-trigger").css("margin-bottom", "2px");
         });
</script>
    
    <script type="text/javascript">
            $(function () {
                $("[id*=txtdate2]").datepicker({
                    showOn: 'button',
                    buttonImageOnly: true,
                    buttonImage: '/Images/calendar.png',
                });
                $(".ui-datepicker-trigger").css("margin-bottom", "2px");
            });
</script>
     <script src="https://ajax.googleapis.com/ajax/libs/jquery/1.6/jquery.min.js" type="text/javascript"></script>
    <script src="https://ajax.googleapis.com/ajax/libs/jqueryui/1.8/jquery-ui.min.js" type="text/javascript"></script>
	<link href="https://ajax.googleapis.com/ajax/libs/jqueryui/1.8/themes/base/jquery-ui.css" rel="Stylesheet" type="text/css" />

    <script type="text/javascript">
        function copyText() {
        var givenvalue = document.getElementById("<%=txtDate.ClientID%>");
        var displayvalue = document.getElementById("<%=txtdate2.ClientID%>");
        var settedvalue = givenvalue;
        displayvalue.value = settedvalue.value;
    }
</script>


</asp:Content>