your date picker creates the form elements form you as:
DATEPICKEDMonth
DATEPICKEDDay
DATEPICKEDYear
so change the code i provided before to use those form element names instead:
<?php if($_SERVER['REQUEST_METHOD'] == "POST") $_POST["fullDate"] = date("Y-m-d", strtotime($_POST["DATEPICKEDYear"]."/".$_POST["DATEPICKEDMonth"]."/".$_POST["DATEPICKEDDay"])); ?>