use Server side validation for this.
select Date validation type.
for the server variable, click the lightning bolt and select the endDate form element.
for the start date, enter:
<?php echo((isset($_POST["startDate"]))?$_POST["startDate"]:"") ?>
for the end date enter:
<?php echo((isset($_POST["startDate"]))?date("m/d/Y", mktime(0,0,0,date("m",strtotime($_POST["startDate"])),date("d",strtotime($_POST["startDate"]))+5,date("Y",strtotime($_POST["startDate"])))):""); ?>
this code is assuming your start date form element is named "startDate", you will need to adjust the code if you use a different name.