try changing:
value="<?php echo((isset($_GET["invalid"])?ValidatedField("blogupdate","BlogDate"):"".(($row_WADAblog["BlogDate"])?date("d-m-Y",strtotime($row_WADAblog["BlogDate"])):"")."")); ?>"
to:
value="<?php echo((isset($_GET["invalid"])?ValidatedField("blogupdate","BlogDate"):"".(($row_WADAblog["BlogDate"] != "")?date("d-m-Y",strtotime($row_WADAblog["BlogDate"])):date("d-m-Y"))."")); ?>"