line 86 of your page performs the conversion to yyyy-mm-dd format using the date() and strtotime() functions.
."" . $WA_AB_Split . "".((isset($_POST["EventDate"]) && $_POST["EventDate"]!="" )?date("Y-m-d H:i:s", strtotime($_POST["EventDate"])):"")
the problem is in the way strtotime() works. strtotime looks at the separator to determine the date format being used.
it sees the "/" separator as a US date and the "-" separator as a UK date.
to work around this problem, change the following code:
date("Y-m-d H:i:s", strtotime($_POST["EventDate"]))
to:
date("Y-m-d H:i:s", strtotime(str_replace("/","-",$_POST["EventDate"])))