On line 55 of your update page you are using the recordset value, or perhaps I added it like this by mistake. This line of code should be updated to be like this:
$WA_fieldValuesStr = "".((isset($_POST["event_date"]))?date("d-m-Y",*strtotime($_POST["event_date"])):"") ."" . "|" . "".((isset($_POST["event_name"]))?$_POST["event_name"]:"") ."";
This way it will attempt to format the date that the user entered rather than the one from the recordset that might not be valid.