JSFiddle - React, Tailwind, and code Playground

HTML

<link rel="stylesheet" href="http://jqueryui.com/web-base-template/themes/jqueryui.com/style.css">
<link rel="stylesheet" href="http://code.jquery.com/ui/1.9.1/themes/base/jquery-ui.css">
<select id="month-select">
    <option value="12">Sauvegarde 12</option>
    <option value="60">Curatelle 60</option>
</select>
<input id="datepicker"/>
<input id="summe"/>

JavaScript

$(function () {
    $( '#datepicker' )
        .change(function () {
            var month = $('#month-select option:selected').val();
            var date = $(this).datepicker('getDate');
            var day = date.getDate();
            
            date.setDate(date.getDate()+(30*month)); 
            date.setDate(day);
            
            $('#summe').attr('value', $.datepicker.formatDate('mm/dd/yy', date));
        })
        .datepicker();
})