JSFiddle - React, Tailwind, and code Playground

HTML

<form>
  Date of dispatch<br>
  <input type="datetime" name="date_dispatch" id="date_dispatch" value="2016-04-26 10:00:00"><br>
  Expected delivery timne<br>
  <input type="datetime" name="date_delivery" id="date_delivery">
  <br>
  <input type="submit" value="Submit">
</form>

JavaScript

$(document).ready(function(){
  $('#date_dispatch').change(function() {
  	var time = new Date($('#date_dispatch').val());
    time.setTime(time.setHours(time.getHours()+3));
  	$('#date_delivery').val(time.toString());
  });
});