You need to paste the code below to the head section of your formname.html.
It goes directly above the
<script type="text/javascript" src="common/js/form_init.js" data-name="" id="form_init_script"> </script>
Adjust your two date picker ID's inside your form html to dt1 and dt2 so it matches the code. Download the needed files.
jquery-1.9.1.js
jquery-ui.js
Place them in folder at the same level as the form embed script. I also removed the script wrapping the iframe code so it looks like this.
<iframe height="400" style="border:none; background:transparent; overflow:hidden; width:100%;" id="fb_iframe" src="dependentdatepicker/dependentdatepicker.html"> </iframe>

<script src='../jquery-1.9.1.js'></script>
<script src="../jquery-ui.js"></script>
<script>
$(window).load(function(){
$(document).ready(function () {
$("#dt1").datepicker({
dateFormat: "mm/dd/yy",
minDate: 0,
onSelect: function (date) {
var date2 = $('#dt1').datepicker('getDate');
date2.setDate(date2.getDate() 1);
//$('#dt2').datepicker('setDate', date2);
//sets minDate to dt1 date 1
$('#dt2').datepicker('option', 'minDate', date2);
}
});
$('#dt2').datepicker({
dateFormat: "mm/dd/yy",
onClose: function () {
var dt1 = $('#dt1').datepicker('getDate');
console.log(dt1);
var dt2 = $('#dt2').datepicker('getDate');
if (dt2 <= dt1) {
var minDate = $('#dt2').datepicker('option', 'minDate');
$('#dt2').datepicker('setDate', minDate);
}
var oneDay = 24*60*60*1000; // hours*minutes*seconds*milliseconds
var firstDate = new Date($("input[name='date1']").val());
var secondDate = new Date($("input[name='date2']").val());
var diffDays = Math.abs((firstDate.getTime() - secondDate.getTime())/(oneDay));
$("input[name='numberofdays']").val((diffDays).toFixed(0));
$("input[name='numberofdays']").attr('readonly', true);
}
});
});
});
</script>