JSFiddle - React, Tailwind, and code Playground

HTML

Start Date
<input type="text" id="start_date" value=''> 
<br>
End date
<input type="text" id="end_date" value=''>

JavaScript

$(document).ready(function(){
    
    function DateFromString(str){ 
        str = str.split(/\D+/);
        str = new Date(str[2],str[0]-1,(parseInt(str[1])+7));
        return MMDDYYYY(str);
    }
    
    function MMDDYYYY(str) {
        var ndateArr = str.toString().split(' ');
        var Months = 'Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec';
        return (parseInt(Months.indexOf(ndateArr[1])/4)+1)+'/'+ndateArr[2]+'/'+ndateArr[3];
    }

    function Add7Days() {
        var date = $('#start_date').val();
        var ndate = DateFromString(date);
        return ndate;
    }

    $('#start_date').change(function(){
        $('#end_date').val(Add7Days());
    });

});