JSFiddle - React, Tailwind, and code Playground

HTML

<div class="form-group">
            	<label>Date:</label>
                <input class="form-control input-lg parsley-validated" type="date" data-required="true" data-type="dateIso" data-date-format='dd-mm-yyyy' name="date" id="date" >
            	</div>
<div class="form-group pull-in clearfix">
                  <div class="col-sm-6"> 
                  <label>Terms:</label>
		          <select id="terms" name="terms" class="form-control parsley-validated input-lg" data-required="true">
                  	<option value="0">Choose Due Date Terms</option>
                    <option value="15">NET 15</option>
                    <option value="30">NET 30</option>
                    <option value="60">NET 60</option>
                  </select>
                  </div>
                  <div class="col-sm-6"> 
                  <label>Due Date:</label>
                  <input class="form-control input-lg parsley-validated" type="date" data-required="true" data-type="dateIso" data-date-format='dd-mm-yyyy' name="duedate" id="duedate">
                  </div>
            	</div>

JavaScript

$('#terms').on('blur', function() {
        var terms = $("#terms").val();
		var date = new Date();
		date.setDate(date.getDate() + +terms);
		var day = ("0" + date.getDate()).slice(-2);
    	var month = ("0" + (date.getMonth() + 1)).slice(-2);
    	var final = date.getFullYear()+"-"+(month)+"-"+(day);
		$("#duedate").val(final);
    });